Editing Telephone number mapping (section)

==Uses==

===Call forwarding===
[[Image:Call Forwarding with ENUM.jpg|520px|thumb|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 (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. 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.

Subdomains of [[e164.arpa]] are delegated on a country-code basis by the ITU. Each delegation is normally made to a regulatory body designated by the national government for the country code concerned. What happens at a country level is a National Matter. In general the conventional DNS registry-registrar model is used. The national ENUM registry manages and operates the DNS infrastructure and related systems for ''country-code''.e164.arpa. It takes registration requests from registrars who are agents of the end users, the registrants. Registrars are typically VoIP providers and telcos who bundle an ENUM registration as part of a VoIP service package. People using an ENUM-enabled VoIP service can dial the registrant's existing number and be connected to the registrant's VoIP telephone over the Internet instead of using the PSTN. When they call someone who does not use ENUM, calls complete over the Public Switched Telephone Network or PSTN in the usual manner. Support for <code>.e164.arpa</code> varies widely between countries; many do not support it at all.

Alternative ENUM-like registries have also emerged. These services verify PSTN numbers and can be used in addition to or as an alternative to e164.arpa. However, if the registry in which a callee's number is not known by the caller, the choice between registries can create confusion and complexity. 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. It is also possible that if an E.164 number is registered in several of these trees, there can be inconsistencies in the information that is returned. Furthermore, the subscriber "owning" a particular E.164 number may not be aware that their number has been entered into one or more of these alternate ENUM-like trees or what information these alternate trees are returning for their E.164 number.

===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 information|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.