h the Access and Mobility Management Function (AMF) and the UPF to modify the current PDU session with the PDU Session Modification procedure. The result of the PDU Session Modification is the establishment of additional QoS flows to carry voice packets. This procedure happens separately for both the calling and called parties. QoS flow tear down Whichever party ends the call, a BYE message will be generated. In this example, it is the calling party that terminates the call. So, the calling party sends a BYE message through the IMS network, and in response, a 200 OK message to that BYE message is sent from the called party. When the IMS network sees this exchange of the BYE and the 200 OK messages, it knows that resources are no longer required in the 5G access network on both legs of the call: Interworking and voice call continuity Calling party Called party 200 OK Figure 8.9 - SIP signaling exchange for call tear down The following diagram shows the procedure for tearing down the call: Policy rule removal instruction Diameter PDU Session Modification P-CSCF Figure 8.10 - Tearing down the call In step 1, the P-CSCF analyzes the SIP BYE and 200 OK messages, and in step 2, it sends an indication based on Diameter to the PCF that the VoNR session is ended. Therefore, in step 3, the PCF sends an instruction to the SMF to remove the policy rule associated with the media flow. Consequently, in step 4, the SMF will do another PDU Session Modification to tear down the QoS flow. Hence, the QoS flow is removed for the voice packets, and all that remains is the QoS flow for SIP signaling. Subsequently, the call is now torn down. Interworking and voice call continuity Interworking is required between VoNR-capable networks and the other legacy networks due to legacy subscribers, coverage gaps, and utilization of other radio technologies. The subscribers of 2G, 3G, and fixed-lined subscribers on Public Switched Telephone Networks (PSTN) do not support SIP signaling. Therefore, the communication between legacy subscribers and 