Tobias needs to register within his home network in order to be able to originate a call toward his sister. In the example so far he has used the SIP URI sip:tobias@ home1.fr for registration. This is the user identity Tobias uses when he uses IMS services that are not work-related. Nevertheless, Tobias has a whole set of user identities that are registered at his operator in France, which are shown in Table 10.3.
During the initial registration procedure, Tobias can explicitly register only one of these URIs, which in the our example is sip:firstname.lastname@example.org. Nevertheless, the IMS allows implicit and explicit registration of further public user identities:
|Registration set||SIP URI||tel URL|
Some of the above-listed identities might automatically (implicitly) be registered by the network during the initial registration phase.
Others might stay unregistered until Tobias explicitly requests them to be registered.
When receiving the 200 (OK) response for the second REGISTER request, both Tobias's terminal and the P-CSCF discover Tobias's default public user identity, which is received as the first URI in the P-Associated-URI header.
To find out more about the registration status of the other public user identities that are assigned to Tobias, the UE automatically subscribes to the registration-state event information ...