nterrogating CSCF (I-CSFC), and Serving CSFC (S-CSCF). P-CSCF is the first point of contact for home network subscribers in the IMS, and SIP signaling is forwarded through P-CSCF. I-CSCF is responsible for selecting S-CSCF, and it communicates with the Unified Data Management (UDM) for the selection procedure. S-CSCF is generally responsible for processing, routing, and responding to SIP signaling messages. S-CSCF interacts with the Telephony Application Server (TAS) in the call setup procedure. The TAS is an IMS application server that supports supplementary service functionalities such as calling line ID hiding and call diversion: I-CSCF Access IP Multimedia Network Subsystem P-CSCF S-CSCF PSTN/PLMN Figure 8.3 - IMS high-level architecture Other elements included in the IMS are listed here: Media Resource Function (MRF) Interconnection Border Control Function/Transition Gateway (IBCF/TrGW) IMS Application Level Gateway/IMS Access Gateway (IMS-ALG/IMS-AGW) Media Gateway Control Function/IMS Media Gateway (MGCF/IMS-MGW) Breakout Gateway Control Function (BGCF) VoNR initial procedures Before the device can conduct VoNR calls, it must pass through a series of initial procedures. These procedures are as follows: Subscriber provisioning IMS connectivity establishment VoNR initial procedures IMS registration Third-party registration Subscriber provisioning Several network elements must be provisioned appropriately with subscriber information for VoNR services. Some of these elements are the UDM, the Home Subscriber Server (HSS), the TAS and Service Centralization and Continuity (SCC) application servers, the Policy Control Function (PCF), and AAA servers in case of Wi-Fi calling. The information indicating whether a subscriber is permitted access to the IMS for voice services is contained within the subscriber profile in the UDM, which acts as a central repository of subscriber information. The subscriber profile contains appropriate QoS profiles for the subscriber, and it also contains whether roaming is supported. L