Question: In UDM/HSS-based P-CSCF restoration for IMS, what triggers the UE to re-select a P-CSCF and register again with IMS when a P-CSCF failure is detected?
Option 1: Receipt of a new initial registration request from the UE
Option 2: UDM sending a P-CSCF restoration indication to either the SMF (triggering IMS PDU session update) or the AMF (initiating IMS PDU session re-establishment)
Option 3: Automatic timeout and retry mechanism on the UE side without network intervention
Option 4: Direct notification from the failed P-CSCF to the UE
Explanation: According to UDM/HSS-based P-CSCF restoration principles, when a P-CSCF failure is detected and there are no other available registration flows for the terminating UE, the UDM sends a P-CSCF restoration indication to either the SMF (initiating an IMS PDU session update) or the AMF (initiating IMS PDU session re-establishment), prompting the UE to re-select a P-CSCF and register again with IMS, ensuring service continuity.