path: root/doc/rfc/rfc3966.txt

Network Working Group                                     H. Schulzrinne
Request for Comments: 3966                           Columbia University
Obsoletes: 2806                                            December 2004
Category: Standards Track


                   The tel URI for Telephone Numbers

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document specifies the URI (Uniform Resource Identifier) scheme
   "tel".  The "tel" URI describes resources identified by telephone
   numbers.  This document obsoletes RFC 2806.

Table of Contents

   1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.   Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.   URI Syntax. . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.   URI Comparisons . . . . . . . . . . . . . . . . . . . . . . .  6
   5.   Phone Numbers and Their Context . . . . . . . . . . . . . . .  6
        5.1.   Phone Numbers. . . . . . . . . . . . . . . . . . . . .  6
               5.1.1. Separators in Phone Numbers . . . . . . . . . .  7
               5.1.2. Alphabetic Characters Corresponding to Digits .  7
               5.1.3. Alphabetic, *, and # Characters as Identifiers.  7
               5.1.4. Global Numbers. . . . . . . . . . . . . . . . .  7
               5.1.5. Local Numbers . . . . . . . . . . . . . . . . .  8
        5.2.   ISDN Subaddresses. . . . . . . . . . . . . . . . . . .  9
        5.3.   Phone Extensions . . . . . . . . . . . . . . . . . . . 10
        5.4.   Other Parameters . . . . . . . . . . . . . . . . . . . 10
   6.   Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.   Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 11
        7.1.   Why Not Just Put Telephone Numbers in SIP URIs?. . . . 11
        7.2.   Why Not Distinguish between Call Types?. . . . . . . . 11
        7.3.   Why tel. . . . . . . . . . . . . . . . . . . . . . . . 11
        7.4.   Do Not Confuse Numbers with How They Are Dialed. . . . 11



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   8.   Usage of Telephone URIs in HTML . . . . . . . . . . . . . . . 11
   9.   Use of "tel" URIs with SIP (Informative). . . . . . . . . . . 12
   10.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
   11.  Security Considerations . . . . . . . . . . . . . . . . . . . 14
   12.  Changes Since RFC 2806. . . . . . . . . . . . . . . . . . . . 14
   13.  References. . . . . . . . . . . . . . . . . . . . . . . . . . 15
        13.1.  Normative References . . . . . . . . . . . . . . . . . 15
        13.2.  Informative References . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 17

1.  Introduction

   This document defines the URI scheme "tel", which describes resources
   identified by telephone numbers.  A telephone number is a string of
   decimal digits that uniquely indicates the network termination point.
   The number contains the information necessary to route the call to
   this point.  (This definition is derived from [E.164] but encompasses
   both public and private numbers.)

   The termination point of the "tel" URI telephone number is not
   restricted.  It can be in the public telephone network, a private
   telephone network, or the Internet.  It can be fixed or wireless and
   address a fixed wired, mobile, or nomadic terminal.  The terminal
   addressed can support any electronic communication service (ECS),
   including voice, data, and fax.  The URI can refer to resources
   identified by a telephone number, including but not limited to
   originators or targets of a telephone call.

   The "tel" URI is a globally unique identifier ("name") only; it does
   not describe the steps necessary to reach a particular number and
   does not imply dialling semantics.  Furthermore, it does not refer to
   a specific physical device, only to a telephone number.

   As commonly understood, telephone numbers comprise two related but
   distinct concepts: a canonical address-of-record and a dial string.
   We define the concepts below:

   Address-of-record or identifier: The telephone number is understood
      here as the canonical address-of-record or identifier for a
      termination point within a specific network.  For the public
      network, these numbers follow the rules in E.164 [E.164], while
      private numbers follow the rules of the owner of the private
      numbering plan.  Subscribers publish these identifiers so that
      they can be reached, regardless of the location of the caller.
      (Naturally, not all numbers are reachable from everywhere, for a





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      variety of technical and local policy reasons.  Also, a single
      termination point may be reachable from different networks and may
      have multiple identifiers.)

   Dial string: "Dial strings" are the actual numbers, symbols, and
      pauses entered by a user to place a phone call.  A dial string is
      consumed by one or more network entities and understood in the
      context of the configuration of these entities.  It is used to
      generate an address-of-record or identifier (in the sense
      described above) so that a call can be routed.  Dial strings may
      require prepended digits to exit the private branch exchange (PBX)
      the end system is connected to, and they may include post-dial
      dual-tone multi-frequency (DTMF) signaling that could control an
      interactive voice response (IVR) system or reach an extension.
      Dial strings are beyond the scope of this document.

   Both approaches can be expressed as a URI.  For dial strings, this
   URI is passed to an entity that can reproduce the actions specified
   in the dial string.  For example, in an analog phone system, a dialer
   translates the dial string into a sequence of actions such as waiting
   for dial tone, sending DTMF digits, pausing, and generating post-dial
   DTMF digits after the callee picks up.  In an integrated services
   digital network (ISDN) or ISDN user part (ISUP) environment, the
   signaling elements that receive protocol messages containing the dial
   string perform the appropriate protocol actions.  As noted, this
   approach is beyond the scope of this specification.

   The approach described here has the URI specify the telephone number
   as an identifier, which can be either globally unique or only valid
   within a local context.  The dialling application is aware of the
   local context, knowing, for example, whether special digits need to
   be dialed to seize an outside line; whether network, pulse, or tone
   dialling is needed; and what tones indicate call progress.  The
   dialling application then converts the telephone number into a dial
   sequence and performs the necessary signaling actions.  The dialer
   does not have to be a user application as found in traditional
   desktop operating systems but could well be part of an IP-to-PSTN
   gateway.

   To reach a telephone number from a phone on a PBX, for example, the
   user of that phone has to know how to convert the telephone number
   identifier into a dial string appropriate for that phone.  The
   telephone number itself does not convey what needs to be done for a
   particular terminal.  Instructions may include dialling "9" before
   placing a call or prepending "00" to reach a number in a foreign
   country.  The phone may also need to strip area and country codes.





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   The identifier approach described in this document has the
   disadvantage that certain services, such as electronic banking or
   voicemail, cannot be specified in a "tel" URI.

   The notation for phone numbers in this document is similar to that in
   RFC 3191 [RFC3191] and RFC 3192 [RFC3192].  However, the syntax
   differs as this document describes URIs whereas RFC 3191 and RFC 3192
   specify electronic mail addresses.  RFC 3191 and RFC 3192 use "/" to
   indicate parameters (qualifiers).  Since URIs use the forward slash
   to describe path hierarchy, the URI scheme described here uses the
   semicolon, in keeping with Session Initiation Protocol (SIP) URI
   conventions [RFC3261].

   The "tel" URI can be used as a request URI in SIP [RFC3261] requests.
   The SIP specification also inherits the 'subscriber' part of the
   syntax as part of the 'user element' in the SIP URI.  Other protocols
   may also use this URI scheme.

   The "tel" URI does not specify the call type, such as voice, fax, or
   data call, and does not provide the connection parameters for a data
   call.  The type and parameters are assumed to be negotiated either
   in-band by the telephone device or through a signaling protocol such
   as SIP.

   This document obsoletes RFC 2806.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC
   2119, [RFC2119] and indicate requirement levels for compliant
   implementations.

3.  URI Syntax

   The URI is defined using the ABNF (augmented Backus-Naur form)
   described in RFC 2234 [RFC2234] and uses elements from the core
   definitions (appendix A of RFC 2234).

   The syntax definition follows RFC 2396 [RFC2396], indicating the
   actual characters contained in the URI.  If the reserved characters
   "+", ";", "=", and "?" are used as delimiters between components of
   the "tel" URI, they MUST NOT be percent encoded.  These characters
   MUST be percent encoded if they appear in tel URI parameter values.






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   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [RFC2396]) are equivalent to their "% HEX HEX" percent
   encoding.

   The "tel" URI has the following syntax:

   telephone-uri        = "tel:" telephone-subscriber
   telephone-subscriber = global-number / local-number
   global-number        = global-number-digits *par
   local-number         = local-number-digits *par context *par
   par                  = parameter / extension / isdn-subaddress
   isdn-subaddress      = ";isub=" 1*uric
   extension            = ";ext=" 1*phonedigit
   context              = ";phone-context=" descriptor
   descriptor           = domainname / global-number-digits
   global-number-digits = "+" *phonedigit DIGIT *phonedigit
   local-number-digits  =
      *phonedigit-hex (HEXDIG / "*" / "#")*phonedigit-hex
   domainname           = *( domainlabel "." ) toplabel [ "." ]
   domainlabel          = alphanum
                          / alphanum *( alphanum / "-" ) alphanum
   toplabel             = ALPHA / ALPHA *( alphanum / "-" ) alphanum
   parameter            = ";" pname ["=" pvalue ]
   pname                = 1*( alphanum / "-" )
   pvalue               = 1*paramchar
   paramchar            = param-unreserved / unreserved / pct-encoded
   unreserved           = alphanum / mark
   mark                 = "-" / "_" / "." / "!" / "~" / "*" /
                          "'" / "(" / ")"
   pct-encoded          = "%" HEXDIG HEXDIG
   param-unreserved     = "[" / "]" / "/" / ":" / "&" / "+" / "$"
   phonedigit           = DIGIT / [ visual-separator ]
   phonedigit-hex       = HEXDIG / "*" / "#" / [ visual-separator ]
   visual-separator     = "-" / "." / "(" / ")"
   alphanum             = ALPHA / DIGIT
   reserved             = ";" / "/" / "?" / ":" / "@" / "&" /
                          "=" / "+" / "$" / ","
   uric                 = reserved / unreserved / pct-encoded

   Each parameter name ("pname"), the ISDN subaddress, the 'extension',
   and the 'context' MUST NOT appear more than once.  The 'isdn-
   subaddress' or 'extension' MUST appear first, if present, followed by
   the 'context' parameter, if present, followed by any other parameters
   in lexicographical order.

      This simplifies comparison when the "tel" URI is compared
      character by character, such as in SIP URIs [RFC3261].




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4.  URI Comparisons

   Two "tel" URIs are equivalent according to the following rules:

   o  Both must be either a 'local-number' or a 'global-number', i.e.,
      start with a '+'.
   o  The 'global-number-digits' and the 'local-number-digits' must be
      equal, after removing all visual separators.
   o  For mandatory additional parameters (section 5.4) and the 'phone-
      context' and 'extension' parameters defined in this document, the
      'phone-context' parameter value is compared as a host name if it
      is a 'domainname' or digit by digit if it is 'global-number-
      digits'.  The latter is compared after removing all 'visual-
      separator' characters.
   o  Parameters are compared according to 'pname', regardless of the
      order they appeared in the URI.  If one URI has a parameter name
      not found in the other, the two URIs are not equal.
   o  URI comparisons are case-insensitive.

   All parameter names and values SHOULD use lower-case characters, as
   tel URIs may be used within contexts where comparisons are case
   sensitive.

   Section 19.1.4 in the SIP specification [RFC3261] discusses one such
   case.

5.  Phone Numbers and Their Context

5.1.   Phone Numbers

   The 'telephone-subscriber' part of the URI indicates the number.  The
   phone number can be represented in either global (E.164) or local
   notation.  All phone numbers MUST use the global form unless they
   cannot be represented as such.  Numbers from private numbering plans,
   emergency ("911", "112"), and some directory-assistance numbers
   (e.g., "411") and other "service codes" (numbers of the form N11 in
   the United States) cannot be represented in global (E.164) form and
   need to be represented as a local number with a context.  Local
   numbers MUST be tagged with a 'phone-context' (section 5.1.5).

   Implementations MUST NOT assume that telephone numbers have a
   maximum, minimum, or fixed length, or that they always begin with or
   contain certain digits.








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5.1.1.  Separators in Phone Numbers

   Phone numbers MAY contain visual separators.  Visual separators
   ('visual-separator') merely aid readability and are not used for URI
   comparison or placing a call.

   Although it complicates comparisons, this specification retains
   visual separators in order to follow the spirit of RFC 2396
   [RFC2396], which remarks that "A URI often needs to be remembered by
   people, and it is easier for people to remember a URI when it
   consists of meaningful components".  Also, ISBN URNs documented in
   RFC 3187 [RFC3187] use visual separators in a manner similar to this
   specification.

   However, even though ITU-T E.123 [E.123] recommends the use of space
   characters as visual separators in printed telephone numbers, "tel"
   URIs MUST NOT use spaces in visual separators to avoid excessive
   escaping.

5.1.2.  Alphabetic Characters Corresponding to Digits

   In some countries, it is common to write phone numbers with
   alphabetic characters corresponding to certain numbers on the
   telephone keypad.  The URI format does not support this notation, as
   the mapping from alphabetic characters to digits is not completely
   uniform internationally, although there are standards [E.161][T1.703]
   addressing this issue.

5.1.3.  Alphabetic, *, and # Characters as Identifiers

   As called and calling terminal numbers (TNs) are encoded in BCD in
   ISUP, six additional values per digit can be encoded, sometimes
   represented as the hexadecimal characters A through F.  Similarly,
   DTMF allows for the encoding of the symbols *, #, and A through D.
   However, in accordance with E.164, these may not be included in
   global numbers.  Their meaning in local numbers is not defined here,
   but they are not prohibited.

5.1.4.  Global Numbers

   Globally unique numbers are identified by the leading "+" character.
   Global numbers MUST be composed with the country (CC) and national
   (NSN) numbers as specified in E.123 [E.123] and E.164 [E.164].
   Globally unique numbers are unambiguous everywhere in the world and
   SHOULD be used.






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5.1.5.  Local Numbers

   Local numbers are unique only within a certain geographical area or a
   certain part of the telephone network, e.g., a private branch
   exchange (PBX), a state or province, a particular local exchange
   carrier, or a particular country.  URIs with local phone numbers
   should only appear in environments where all local entities can
   successfully set up the call by passing the number to the dialling
   software.  Digits needed for accessing an outside line, for example,
   are not included in local numbers.  Local numbers SHOULD NOT be used
   unless there is no way to represent the number as a global number.

   Local numbers SHOULD NOT be used for several reasons.  Local numbers
   require that the originator and recipient are configured
   appropriately so that they can insert and recognize the correct
   context descriptors.  Since there is no algorithm to pick the same
   descriptor independently, labelling numbers with their context
   increases the chances of misconfiguration so that valid identifiers
   are rejected by mistake.  The algorithm to select descriptors was
   chosen so that accidental collisions would be rare, but they cannot
   be ruled out.

   Local numbers MUST have a 'phone-context' parameter that identifies
   the scope of their validity.  The parameter MUST be chosen to
   identify the local context within which the number is unique
   unambiguously.  Thus, the combination of the descriptor in the
   'phone-context' parameter and local number is again globally unique.
   The parameter value is defined by the assignee of the local number.
   It does NOT indicate a prefix that turns the local number into a
   global (E.164) number.

   There are two ways to label the context:  via a global number or any
   number of its leading digits (e.g., "+33") and via a domain name,
   e.g., "houston.example.com".  The choice between the two is left to
   the "owner" of the local number and is governed by whether there is a
   global number or domain name that is a valid identifier for a
   particular local number.

   The domain name does not have to resolve to any actual host but MUST
   be under the administrative control of the entity managing the local
   phone context.

   A global number context consists of the initial digits of a valid
   global number.  All global numbers with these initial digits must be
   assigned to the same organization, and no such matching number can be
   used by any other organization.  For example, +49-6151-16 would be a
   suitable context for the Technical University of Darmstadt, as it
   uses all numbers starting with those digits.  If such an initial



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   string of digits does not exist, the organization SHOULD use the
   lowest number of the global number range assigned to it.  (This can
   occur if two organizations share the same decimal block of numbers.
   For example, assume an organization owns the number range +1-212-
   555-0100 through +1-212-555-0149.  +1-212-555-1 would not be a valid
   global number context, but +1-212-555-0100 would work.) It is not
   required that local numbers within the context actually begin with
   the chosen set of initial numbers.

   A context consisting of the initial digits of a global number does
   not imply that adding these to the local number will generate a valid
   E.164 number.  It might do so by coincidence, but this cannot be
   relied upon.  (For example, "911" should be labeled with the context
   "+1", but "+1-911" is not a valid E.164 number.)

   National freephone numbers do not need a context, even though they
   are not necessarily reachable from outside a particular country code
   or numbering plan.  Recall that "tel" URIs are identifiers; it is
   sufficient that a global number is unique, but it is not required
   that it be reachable from everywhere.

      Even non-freephone numbers may be out of date or may not be
      reachable from a particular location.  For example, premium
      services such as "900" numbers in the North American numbering
      plan are often not dialable from outside the particular country
      code.

      The two label types were chosen so that, in almost all cases, a
      local administrator can pick an identifier that is reasonably
      descriptive and does not require a new IANA-managed assigned
      number.  It is up to the administrator to assign an appropriate
      identifier and to use it consistently.  Often, an organization can
      choose among several different identifiers.

   If the recipient of a "tel" URI uses it simply for identification,
   the receiver does not need to know anything about the context
   descriptor.  It simply treats it as one part of a globally unique
   identifier, with the other being the local number.  If a recipient of
   the URI intends to place a call to the local number, it MUST
   understand the context and be able to place calls within that
   context.

5.2.  ISDN Subaddresses

   A phone number MAY also contain an 'isdn-subaddress' parameter that
   indicates an ISDN subaddress.





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   ISDN subaddresses typically contain International Alphabet 5 (IA5
   [T.50]) characters but may contain any octet value.

5.3.  Phone Extensions

   Phone extensions identify stations behind a non-ISDN PBX and are
   functionally roughly equivalent to ISDN subaddresses.  They are
   identified with the 'extension' parameter.  At most, one of the
   'isdn-subaddress' and 'extension' parameters can appear in a "tel"
   URI, i.e., they cannot appear both at the same time.

5.4.  Other Parameters

   Future protocol extensions to this URI scheme may add other
   parameters ('parameter' in the ABNF).  Such parameters can be either
   mandatory or optional.  Mandatory parameters start with "m-".  An
   implementation MAY ignore optional parameters and MUST NOT use the
   URI if it contains unknown mandatory parameters.  The "m-" prefix
   cannot be added to parameters that were already registered (except to
   create a new, logically distinct parameter).  The "phone-context"
   parameter in this document is mandatory,  and "isub" and "ext" are
   optional.

   New mandatory parameters must be described in a standards-track RFC,
   but an informational RFC is sufficient for optional parameters.

   For example, 'parameter' parameters can be used to store
   application-specific additional data about the phone number, its
   intended use, or any conversions that have been applied to the
   number.

   Entities that forward protocol requests containing "tel" URIs with
   optional parameters MUST NOT delete or modify parameters they do not
   understand.

6.  Examples

   tel:+1-201-555-0123: This URI points to a phone number in the United
      States.  The hyphens are included to make the number more human
      readable; they separate country, area code and subscriber number.

   tel:7042;phone-context=example.com: The URI describes a local phone
         number valid within the context "example.com".

   tel:863-1234;phone-context=+1-914-555: The URI describes a local
      phone number that is valid within a particular phone prefix.





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7.  Rationale

7.1.  Why Not Just Put Telephone Numbers in SIP URIs?

   The "tel" URI describes a service, reaching a telephone number, that
   is independent of the means of doing so, be it via a SIP-to-PSTN
   gateway, a direct SIP call via E.164 number ("ENUM") translation
   [RFC3761], some other signaling protocols such as H.323, or a
   traditional circuit-switched call initiated on the client side via,
   say, the Telephony Application Programming Interface (TAPI).  Thus,
   in spirit, it is closer to the URN schemes that also leave the
   resolution to an external mechanism.  The same "tel" URI may get
   translated to any number of other URIs in the process of setting up
   the call.

7.2.  Why Not Distinguish between Call Types?

   Signaling protocols such as SIP allow negotiating the call type and
   parameters, making the very basic indication within the URI scheme
   moot.  Also, since the call type can change frequently, any such
   indication in a URI is likely to be out of date.  If such designation
   is desired for a device that directly places calls without a
   signaling protocol such as SIP, mechanisms such as the "type"
   attribute for the "A" element in HTML may be more appropriate.

7.3.  Why "tel"?

   "tel" was chosen because it is widely recognized that none of the
   other suggestions appeared appropriate.  "Callto" was discarded
   because URI schemes locate a resource and do not specify an action to
   be taken.  "Telephone" and "phone" were considered too long and not
   easily recognized internationally.

7.4.  Do Not Confuse Numbers with How They Are Dialed

   As an example, in many countries the E.164 number "+1-212-555-3141"
   will be dialed  as 00-1-212-555-3141, where the leading "00" is a
   prefix for international calls.  (In general, a "+" symbol in E.164
   indicates that an international prefix is required.)

8.  Usage of Telephone URIs in HTML

   Links using the "tel" URI SHOULD enclose the telephone number so that
   users can easily predict the action taken when following the link

   Dial <a href="tel:+1-212-555-0101">+1-212-555-0101</a> for
   assistance.




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   instead of

   Dial <a href="tel:+1-212-555-0101">this number</a> for assistance.

   On a public HTML page, the telephone number in the URI SHOULD always
   be in the global form, even if the text of the link uses some local
   format:

   Telephone (if dialling in the United States):
     <a href="tel:+1-201-555-0111">(201) 555-0111</a>

   or even

   For having RFCs read aloud, call <a
   href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.

9.  Use of "tel" URIs with SIP (Informative)

   SIP can use the "tel" URI anywhere a URI is allowed, for example as a
   Request-URI, along with "sip" and "sips" URIs.  For brevity, we will
   imply "sips" URIs when talking about SIP URIs.  Both "tel" and SIP
   URIs can contain telephone numbers.  In SIP URIs, they appear as the
   user part, i.e., before the @ symbol (section 19.1.6 in [RFC3261]).

   Unless a SIP UA connects directly to a PSTN gateway, one of the SIP
   proxy servers has to translate the "tel" URI to a SIP URI, with the
   host part of that URI pointing to a gateway.  Typically, the outbound
   proxy server, as the first proxy server visited by a call request,
   performs this translation.  A proxy server can translate all "tel"
   URIs to the same SIP host name or select a different gateway for
   different "tel" prefixes, based, for example, on information learned
   from TRIP [RFC3219].  However, a proxy server could also delegate
   this translation task to any other proxy server, as proxy servers are
   free to apply whatever routing logic they desire.  For local numbers,
   the proxy MUST NOT translate "tel" URIs whose contexts it does not
   understand.

   As noted earlier, all phone numbers MUST use the global form unless
   they cannot be represented as such.  If the local-number format is
   used, it MUST be qualified by the 'phone-context' parameter.
   Effectively, the combination of local number and phone context makes
   the "tel" URI globally unique.

   Although web pages, vCard business cards, address books, and
   directories can easily contain global "tel" URIs, users on twelve-
   button (IP) phones cannot dial such numbers directly and are
   typically accustomed to dialling shorter strings, e.g., for PBX
   extensions or local numbers.  These so-called dial strings (section



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   1) are not directly represented by "tel" URIs, as noted.  We refer to
   the rules that govern the translation of dial strings into SIP and
   "tel" URIs, global or local, as the dial plan.  Currently,
   translations from dial strings to "tel" URIs have to take place in
   end systems.  Future efforts may provide means to carry dial strings
   in a SIP URI, for example, but no such mechanisms exist as of this
   writing.

   A SIP UA can use a dial plan to translate dial strings into SIP or
   "tel" URIs.  The dial plan can be manually configured or, preferably,
   downloaded as part of a device configuration mechanism.  (At this
   time, there is no standardized mechanism for this.)

   A mobile user can use at least two dial plans, namely the dial plan
   for the network that he is currently visiting and the dial plan for
   his home network.  Generally, dialed numbers meant to represent
   global numbers will look the same after the translation regardless of
   the dial plan, even if, say, the visited network uses '0' for
   dialling an 'outside' number and the user's home network uses '9', as
   long as the user is aware of the current dial plan.  However, local
   extensions without a direct global equivalent may well behave
   differently.  To avoid any ambiguity, the dial plan MUST insert a
   suitable 'phone-context' string when performing the translation.  If
   the 'phone-context' is a domain name, there are three cases:

   1.  The outbound proxy recognizes the domain name in the "tel" or SIP
       URI as its local context and can route the request to a gateway
       that understands the local number.

   2.  The outbound proxy does not use the same phone context but can
       route to a proxy that handles this phone context.  This routing
       can be done via a lookup table, or the domain name of the phone
       context might be set up to reflect the SIP domain name of a
       suitable proxy.  For example, a proxy may always route calls with
       "tel" URIs like

          tel:1234;phone-context=munich.example.com

       to the SIP proxy located at munich.example.com.  (Proxies
       receiving a tel URI with a context they do not understand are
       obligated to return a 404 (Not Found) status response so that an
       outbound proxy may decide to attempt such a heuristic.)

   3.  The outbound proxy does not recognize the phone context and
       cannot find the appropriate proxy for that phone context.  In
       that case, the translation fails, and the outbound proxy returns
       a 404 (Not Found) error response.




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10.  Acknowledgments

   This document is derived from RFC 2806 [RFC2806], written by Antti
   Vaehae-Sipilae.  Mark Allman, Flemming Andreasen, Francois Audet,
   Lawrence Conroy, Cullen Jennings, Michael Hammer, Paul Kyzivat,
   Andrew Main, Xavier Marjou, Jon Peterson, Mike Pierce, Jonathan
   Rosenberg, and James Yu provided extensive comments.

11.  Security Considerations

   The security considerations parallel those for the mailto URL
   [RFC2368].

   Web clients and similar tools MUST NOT use the "tel" URI to place
   telephone calls without the explicit consent of the user of that
   client.  Placing calls automatically without appropriate user
   confirmation may incur a number of risks, such as those described
   below:

   o  Calls may incur costs.
   o  The URI may be used to place malicious or annoying calls.
   o  A call will take the user's phone line off-hook, thus preventing
      its use.
   o  A call may reveal the user's possibly unlisted phone number to the
      remote host in the caller identification data and may allow the
      attacker to correlate the user's phone number with other
      information, such as an e-mail or IP address.

   This is particularly important for "tel" URIs embedded in HTML links,
   as a malicious party may hide the true nature of the URI in the link
   text, as in

   <a href="tel:+1-900-555-0191">Find free information here</a>
   <a href="tel:+1-900-555-0191">tel:+1-800-555-0191</a>


   "tel" URIs may reveal private information, similar to including phone
   numbers as text.  However, the presence of the tel: schema identifier
   may make it easier for an adversary using a search engine to discover
   these numbers.

12.  Changes Since RFC 2806

   The specification is syntactically backwards-compatible with the
   "tel" URI defined in RFC 2806 [RFC2806] but has been completely
   rewritten.  This document more clearly distinguishes telephone
   numbers as identifiers of network termination points from dial
   strings and removes the latter from the purview of "tel" URIs.



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   Compared to RFC 2806, references to carrier selection, dial context,
   fax and modem URIs, post-dial strings, and pause characters have been
   removed.  The URI syntax now conforms to RFC 2396 [RFC2396].

   A section on using "tel" URIs in SIP was added.

13.  References

13.1.  Normative References

   [E.123]    International Telecommunications Union, "Notation for
              national and international telephone numbers,  e-mail
              addresses and web addresses", Recommendation E.123,
              February 2001.

   [E.161]    International Telecommunications Union, "Arrangement of
              digits, letters and symbols on telephones and other
              devices that can be used for gaining access to a telephone
              network", Recommendation E.161, May 1995.

   [E.164]    International Telecommunications Union, "The international
              public telecommunication numbering plan", Recommendation
              E.164, May 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP:  Session Initiation Protocol", RFC 3261,
              June 2002.

   [T1.703]   ANSI, "Allocation of Letters to the Keys of Numeric
              Keypads for Telecommunications", Standard T1.703-1995
              (R1999), 1999.













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13.2.  Informative References

   [RFC2368]  Hoffman, P., Masinter, L., and J. Zawinski, "The mailto
              URL scheme", RFC 2368, July 1998.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2806]  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
              April 2000.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187, October
              2001.

   [RFC3191]  Allocchio, C., "Minimal GSTN address format in Internet
              Mail", RFC 3191, October 2001.

   [RFC3192]  Allocchio, C., "Minimal FAX address format in Internet
              Mail", RFC 3192, October 2001.

   [RFC3219]  Rosenberg, J., Salama, H., and M. Squire, "Telephony
              Routing over IP (TRIP)", RFC 3219, January 2002.

   [T.50]     International Telecommunications Union, "International
              Reference Alphabet (IRA) (Formerly International Alphabet
              No. 5 or IA5) - Information technology - 7-bit coded
              character set for information interchange", Recommendation
              T.50, 1992.

Author's Address

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   US

   Phone: +1 212 939 7042
   EMail: hgs@cs.columbia.edu
   URI:   http://www.cs.columbia.edu




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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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