Telephone number mapping

It uses special DNS record types to translate a telephone number into a Uniform Resource Identifier (URI) or IP address that can be used in Internet communications.

The problem of a single universal personal identifier for multiple communication services can be solved with different approaches.

ENUM also permits connecting the IP world to the telephone system in a seamless manner.

This URI will be used for looking-up and fetching 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 3403), 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.

RFC 6116 defines how any E.164 number, such as +1 555 42 42 can be transformed into a URI, by reversing the numbers, separating them with dots and adding the e164.arpa suffix thus: 2.4.2.4.5.5.5.1.e164.arpa The URI can then be used for obtaining the Internet Protocol addresses for services such as the Session Initiation Protocol (SIP) VoIP telephony.

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.

The caller uses the telephone to dial the number of another subscriber, which leads to an ENUM lookup (such as is provided by SIP Broker).

The DNS responds to the caller by returning a list with NAPTR records for VoIP communication, telephone numbers and email addresses.

Each delegation is normally made to a regulatory body designated by the national government for the country code concerned.

Multiple DNS lookups may be needed and it is far from simple to know which E.164 numbers are registered in which of these alternate ENUM-like trees.

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.

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.