n procedure, while the UDM has the security information together with the subscriber profile. The IMS registration procedure is outlined in Figure 7.4. In step 1, the device sends an initial registration request to the IMS. This request contains information such as the device capabilities and identification information. Then in step 2, the P-CSCF obtains the security information from the UDM. That security information allows the IMS network to send an authentication challenge down to the device. Finally, in step 3, S-CSCF sends the authentication challenge to the device. Now, in step 4, the device attempts to register again with the authentication response to the challenge in step 2 and 3. If the response is correct, then in step 5, S-CSCF allows the device to access the IMS services: P-CSCF S-CSCF Figure 8.5 - The IMS registration procedure In terms of the IMS, the network is now set up to receive and process any mobile-originated or terminated SIP signaling requests. Third-party registration Registration to the application servers such as the TAS is not part of the IMS registration procedure. However, in the IMS registration procedure, S-CSCF interacts with the UDM and downloads the user profile in the form of Initial Filter Criteria (IFC), which comprises a list of conditions upon which the application servers can be registered on behalf of the subscriber: VoNR call procedures Third-Party Registration S-CSCF Figure 8.6 - Third-party registration Therefore, following the IMS registration procedure, S-CSCF sends a third-party registration to the application servers listed in the IFC content. VoNR call procedures This section evaluates the activities that need to take place to ultimately set up a VoNR call. High-level call procedure activities Key procedures take place within the 5G system, IMS, and policy charging control elements such as PCF to make a VoNR call. There is activity taking place with the SIP and the IMS. There is also activity taking place with the Diameter protocol and PCF. Moreover, there is als