ENUM refers to "E.164 Number Mapping". It is a protocol that is responsible for mapping between regular telephone numbers (E.164 format), and the SIP URI of the IMS subscribers in the IMS network.

In order to perform this task, the IMS network uses the 'ENUM server' that coexists with the DNS server in the IMS network.

Using this ENUM server, when the IMS network receives a voice or a video call, the IMS network will be able to know if the recipient's phone number is an IMS subscriber or not.

This helps the IMS network to decide if it will route the call through the IMS network, or through the CS network.

This ENUM server contains an 'ENUM registry' that stores the telephone numbers of all IMS subscribers in the network.

When a user needs to initiate a communication session (e.g. voice or video call), the IMS network performs recipient's telephone number lookup with the ENUM server to find the corresponding SIP URI of the recipient's phone number.

If the recipient's telephone number is an IMS subscriber, the ENUM server will be able to resolve its regular telephone number to an SIP URI for this subscriber. And then returns this SIP URI to the IMS network.

While if the recipient's telephone number is not an IMS subscriber, the ENUM server will not be able to resolve the recipient's telephone number to a SIP URI. So that the ENUM will return an error to the IMS network. So that the IMS network knows that this recipient's telephone number is not an IMS subscriber.