IMS registration over 5G

IMS Registration Prerequisites

Before making or receiving VoNR calls, the UE must register with IMS. This happens after 5G NR registration and IMS PDU session establishment. First, the UE establishes an IMS PDU session with default QoS (5QI=5 for SIP signaling). During PDU Session Establishment Accept, the UE discovers the P-CSCF address via the PCO (Protocol Configuration Options) field. With the signaling path ready, the UE sends its first SIP REGISTER through P-CSCF to the home network's I-CSCF. The I-CSCF queries UDM via the Cx interface to get the assigned S-CSCF address, then forwards the REGISTER.