Question: What triggers a P-CSCF to subscribe to notifications of PLMN ID changes for a UE's location?
Option 1: Receipt of an initial REGISTER SIP message from an attached UE
Option 2: Receiving an AA-Answer or RA-Request from the PCRF without prior UE registration
Option 3: Periodic network scans for UE location updates
Option 4: User initiation of a voice call via the IM CN subsystem
Explanation: The initial REGISTER SIP message from a UE signifies its attachment to the network, prompting the P-CSCF to subscribe to PLMN ID change notifications for effective session management across potential network changes, ensuring seamless communication services as the UE moves between different PLMNs.