Uses

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.

ITU e164.arpa 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 ENUM services such as E164.org have also emerged. These services often verify PSTN numbers and can be used in addition to e164.arpa. Competing ENUM zones may be seen as beneficial, to keep prices low in the VoIP market. 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.