ENUM brings your telephone number on the internet and provides the connection of internet services with this number. In addition to the traditional telephone service, numbers can also be used for addressing Voice over IP (VoIP), Instant Messaging, Presence, e-mail, web and other services.

One example for one of the most important applications is the addressing of VoIP services from the PSTN:
While telephone numbers within the traditional telephone system only consist of numbers, URI can include numbers as well as letters and special characters.
Here is an example for a “telephone number” of a VoIP service:
“sip:user001@server.enum.at“

If a traditional phone user wants to call this type of numbers, there is the imminent problem of dialling this address on a number keypad. With the aid of ENUM the complexity of an internet address can be hidden behind a normal telephone number. Therefore three steps are necessary:

1.) The VoIP participant registers an ENUM domain which is assigned to his telephone number, for example the number of his phone extension, and defines the services he wants to be reachable via VoIP.

2.) The caller uses an intelligent end device (e.g. a private branch exchange or the gateway of a telecom provider), which queries the relevant central database of the number called whether there are any internet-based services available.

3.) If a relevant service exists, the connection is established via internet – otherwise via PSTN.

The result is that the caller still dials a telephone number on his end device, just as usual. Thus, he doesn’t have to change his customs or worry about the details regarding the addressing or the establishment of the connection.

ENUM itself is not a gateway service that links the traditional telephone network with the internet, but it is rather a technology which simplifies this connection by using common means of addressing – just imagine it as some kind of information database for intelligent systems.

The telephone example mentioned above is only one of the possible applications. ENUM can also be used for addressing fax or Instant Messaging services. However, the basic mode of operation is always the same: One or more services are specified within the ENUM database for one telephone number from the official number network – namely one number which is also used in the fixed and mobile telephone network. The configuration of these records is subject to the person who has the usage rights of the respective call number. These records are publicly available for anyone.