ignment-Answer) message. The mobile profile contains the logic to invoke the telephony application server (TAS) that provides supplementary telephone services and authorized media. The S-CSCF entity responds to the mobile with a SIP 200 OK message including its identity in the Service Route header. The registration is effective for a duration indicated in the parameter expires in the header Contact. The mobile has to renew its registration before expiration of that time, by way of the same procedure as for initial registration. The S-CSCF entity indicates, in the header P-Associated-URI, the identities registered implicitly, in addition to that indicated in the header to. SIP/2.0 200 OK Contact: <sip : 192.0.2.101>;expires=600000 P-Associated-URI: <sip:+1-212-555-1111@homeA.net user=phone> VoLTE and ViLTE 18), 19), 20) The SIP 200 OK response follows the reverse path taken by the REGISTER request, through the Via headers. 21) The S-CSCF entity registers the mobile with the TAS entity. 22) The TAS entity sends to the HSS entity the DIAMETER UDR (User- Data-Request) message to retrieve the mobile service data. 23) The answer is provided by the DIAMETER UDA (User-Data- Answer) message. 24) The TAS entity responds with 200 OK message to the SIP REGISTER request. 25) Alice's UA entity (or Bob's) generates a SUBSCRIBE request to receive notifications for registration. The identity of the S-CSCF entity of the domain (homeA. net), contained in the header Route, is learned during the registration of Alice's UA entity, information from the header Service Route. If Alice's UA entity has multiple identities, it indicates in the header P -Preferred-Identity which among them is the preferred one (sip:alice@homeA.net). Alice's UA entity defines in the header Event the type of event which it wishes to subscribe (value reg for registration). Alice's UA entity publishes the duration of the subscription in the header Expires. The value must not be greater than that indicated during registration in the response 200 OK of the REGISTE