Telephone Number Mapping

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Telephone Number Mapping (ENUM or Enum, from TElephone NUmber Mapping) is a suite of protocols to unify the telephone numbering system E.164 with the Internet addressing system DNS by using an indirect lookup method, using the Enum telephone number as a key to obtain NAPTR records. The records are stored at a DNS database.

Although it facilitates calling VOIP users from IP and PSTN networks, ENUM is not a VOIP function and should not be confused with common VOIP routing based on SIP and H.323 protocols with a Uniform Resource Identifier (URI).


Being able to dial telephone calls the way customers have come to expect is considered crucial for the convergence of classic telephone service (PSTN) and Internet telephony (VOIP), and for the development of new IP multimedia services. The problem of a single universal personal identifier for multiple communication services can be solved with different approaches. One simple approach is the Electronic Number Mapping System ENUM (also known as Telephone Number Mapping), deve­lop­­ed by the IETF, using existing E.164 telephone numbers, protocols and infrastructure to indirectly access different services available under a single personal identifier. ENUM also permits communica­ting the IP world to the telephone system in a seamless manner.

ENUM varieties

One potential source of confusion, when talking about ENUM, is the variety of ENUM implementations in place today. Quite often, people speaking of ENUM are really referring to only one of the following:

  • Public ENUM: The original vision of ENUM as a global, public directory-like database, with subscriber opt-in capabilities and delegation at the country code level in the domain. This is also referred to as User ENUM.
  • Private ENUM: A carrier, VOIP operator or ISP may use ENUM techniques within its own networks, in the same way DNS is used internally to networks.
  • Carrier ENUM: Groups of carriers or communication service providers agree to share subscriber information via ENUM in private peering relationships. The carriers themselves control subscriber information, not the individuals. Carrier ENUM is also referred to as Infrastructure ENUM, and is being the subject of new IETF recommendations to support VOIP peering.

Public ENUM System details

An ENUM subscriber to be able to activate and use the ENUM service needs to obtain three elements from a Registrar:

  1. A personal Uniform Resource Identifier (URI) to be used on the IP part of the network, as explained below
  2. One E.164 regular personal telephone number associated with the personal URI, to be used on the PSTN part of the network
  3. Authority to write his/her call forwarding/termination preferences in the NAPTR record accessible via the personal URI

This works as follows: (1) the Registrar provides the Subscriber (or Registrant) a domain name, the URI, that will be used to access a DNS server to fetch a NAPTR record, (2) a personal E.164 telephone number (the ENUM number). The URI domain name of (1) is biunivocally associated to the subscriber E.164 ENUM number of (2). Finally (3) the NAPTR record corresponding to the subscriber URI contains the subscriber call forwarding/termination preferences.

Therefore, if a calling party being at the PSTN network dials a called party ENUM number by touch typing the E.164 called party number, the number will be translated at the ENUM gateway into the corresponding URI. This URI will be used to look-up and fetch the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (either on IP or on PSTN terminations) – the so-called access information – which the registrant (the called party) has specified by writing his/her choice at the ‘NAPTR record’, "Naming Authority Pointer Resource Records" as defined in RFC 2915, such as e-mail addresses, a fax number, a personal website, a VOIP number, mobile telephone numbers, voice mail systems, IP-telephony addresses, web pages, GPS coordinates, call diversions or instant messaging. Alternately, when the calling party is at the IP side, the User Agent (UA) piece of software of the dialler will allow to dial a E.164 number, but the dialler UA will convert it into a URI, to be used to look-up at the ENUM gateway DNS and fetch the NAPTR record obtaining the called party wishes about how the call should be forwarded or terminated (again, either on IP or on PSTN terminations).

Calling by using a new personal E.164 number (the ENUM number) to look-up at a database is therefore an indirect calling support service.

The ITU ENUM allocates a specific zone, namely "" for use with ENUM E.164 numbers on the IP side of the network. RFC 3761 define how any ENUM number, such as +1 555 42 42 can be transformed into a URI, by reversing the numbers, separating them with dots and adding the suffix thus:

The URI can then be used to look up at the DNS the Internet addresses for services such as SIP VOIP telephony. NAPTR records are used to set the subscriber call forwarding/termination preferences. Therefore the whole system can 'translate' E.164 addresses to SIP addresses. An example NAPTR record is:

IN NAPTR 100 10 "u" "E2U+sip"  "!^.*$!!" .
IN NAPTR 102 10 "u" "E2U+mailto" "!^.*$!!" .

This means the holder of number +1 555 4242 can be called via SIP at and via email at

Therefore, to use the "E2U+sip" service, callers should dial +1 555 4242 and the system will use as the address. The regular expression can be used by a telephone company to easily assign addresses to all of its clients. For example, if your number is +15554242, your SIP address would be; if your number is +15551234, your SIP address would be

The following Figure illustrates how ENUM works by giving an example: Subscriber A sets out to call Subscriber B.

Application scenario for Voice over IP (VOIP) with ENUM.

  1. The User Agent of an ENUM-enabled subscriber terminal device, or a PBX, or a Gateway, translates the request for the number +34 98 765 4321 in accordance with the rule described in RFC 3761 into the ENUM domain
  2. A request is sent to the Domain Name System DNS asking it to look up the ENUM domain
  3. The query returns a result in the form of so called Naming Authority Pointer Resource NAPTR records, as per RFC 3403. In the example above, the response is an address that can be reached in the Internet using the VOIP protocol, SIP per RFC 3261.
  4. The terminal application now sets up a communication link, and the call is routed via the Internet.

The ENUM user does not notice anything of this reversal and the DNS database look-up, as this is done automatically behind the scene using a user agent software in his PC or terminal, or at the PABX or Gateway. For instance, when the user types the telephone number in his web browser ENUM enabled agent and indicates what item of information he is looking for (email address, telephone number, web address, etc.) in the PC or terminal the number is converted to a domain name. This is sent to ENUM servers on the Internet, which send back the NAPTR records associated with the name. The access information and any priority indicated for them are stored in these. The user gets the requested address back on his PC or terminal. ENUM therefore in fact functions as a mechanism for translating a telephone number into a domain name with the requested address or number associated with it, but without the user viewing how this is done, just as he is currently unaware that he is using the DNS when he makes a connection with the Internet or what is going on at the telephone switch when he makes a call.


Call forwarding with ENUM

One way of doing call forwarding with ENUM is illustrated in the next figure. The caller uses the telephone to dial the number of another subscriber, which leads to an ENUM lookup. The DNS responds to the caller by returning a list with NAPTR records for VOIP communication, telephone numbers and email addresses. Next, an attempt will be made, using the VOIP record from this list, to establish a connection with the subscriber. If the subscriber is not online, the next record selected will be that for a connection to a PSTN or mobile telephone. If this attempt fails too, a voice message will be sent to the subscriber via a listed email address.

Call forwarding with ENUM.

ITU subdomains are first delegated to ("registered by") regulatory bodies designated by the national government of the country code concerned, which further delegates zones to telecommunications providers. Your telephone company is therefore in charge of the NAPTR records, usually. Some countries are proposing to let end-users register their own telephone numbers via an intermediary, which need not be their own telco. This is considered a good idea as VOIP is a major Enum use. People who use an Enum-enabled VOIP service can dial your existing number and be connected not to your existing phone line, but to your own VOIP telephone directly via the Internet, bypassing the telephone system entirely. When they call someone who does not use Enum, calls complete over the Public Switched Telephone Network or PSTN.

Alternative free public and private ENUM services such as and VPF ENUM Registry have also emerged. These services use PSTN numbers to look-up termination URIs via private NAPTR records and can be used in addition to Also other ENUM lookup protocols such as sbXML are also available. These services claim to provide a faster and easier mechanism to translate E.164 numbers into SIP addresses.

Called party facility

ENUM can also be viewed as a called party facility. Basically, it is an indirect dialling service designed to work seamlessly on PSTN and VOIP that builds on the great value of the e.164 numbers: billions of people knowing how to dial using numbers.

If the called person has opted to use ENUM she/he will have published the ENUM number and have entered (via ENUM NAPTR) his/her wishes for how the call should be terminated. This might be a single VOIP identifier, but most likely it will be a list of how the call should be forwarded to various fixed-line, cellphones, secretarial or voice mail services, either at the IP or at the PSTN side of the network. It is the called party choice to opt-in ENUM and also to decide to let the calling party know her/his wishes.

Today when a user places a regular phone call, he has to begin deciding how to establish the call with the other party: via VOIP, Fixed-line PSTN, cellphone… entering a URI or dialling a number. With ENUM indirect dialling it is the called party wishes that matter and solve that decision. Another benefit of indirect dialling is to free the user to change his phone telco, webpage, IMS, email or whatever telecom service he uses without having to tell all his contacts about that.

A presence enhanced ENUM facility having various profiles could automatically change the called party wishes as a function of where he/she is available. This could be a mechanism to automatically switch between cellphone and VOIP to the most convenient (or the less costing) termination.

Parties having a direct interest in ENUM

Various parties are involved with ENUM. These include:

The registrant or subscriber
The registrant is the person or subscriber that makes his access information available to others through ENUM. The registrant or subscriber is thus the person whose information has been included in ENUM and must not be confused with the person who uses the Internet to find an address through ENUM.
The registrar
The registrar is the party who manages the registrant’s access information and ensures that it is publicly available on the Internet.
The registry
The registry is the manager of the National ENUM zone, or . The registry forms, as it were, the top of the National ENUM pyramid and ensures that reference is made to the registrars’ servers on which the access information is located.
Because of the hierarchical structure of the DNS, there can only be one registry for every National ENUM zone. To prevent abuse of this position, requirements are laid down regarding the impartiality of the registry and the costs and quality of its service. In addition every registrant must have equal and open access.
The government or the regulator
Usually a governmental entity or a regulatory authority has control over the National zone of ENUM and will play a role in the appointment of the registry.
The number holder operator
Telephony services or telecommunication services operators have being assigned blocks of numbers by the regulator. They subsequently enable their users to use individual telephone numbers from those number blocks. Examples are the numbers for fixed telephony and mobile telephony. The number holder operator will be interconnected to other operators and will receive from them calls to his assigned range of numbers, for the calls to be terminated.
In ENUM the number holder operator will typically be the gateway operator or, alternatively, will have an arrangement with a gateway operator, to whom he will transit the calls.
But ENUM is a personal number, meant to be valid for the registrant life. Consequently in ENUM once the operator number holder assigns a number to a registrant, the number belongs to that registrant during his/her entire life.
Hence, if the registrant wishes to change his initial number holder operator (that might also coincide being his gateway operator) there have to be provisions for the ENUM number to be ported from the initial operator to other number holder operators.