Question: In an IMS emergency registration or session establishment scenario without IMS-level roaming interfaces, what triggers the P-CSCF to request EPC-level identities (MSISDN, IMSI, IMEI(SV)) from the PCRF?
Option 1: Receipt of an initial REGISTER SIP message with a specific emergency service indicator.
Option 2: Determination of no IMS-level roaming interfaces and absence of service information in the AF.
Option 3: AA-Answer command from PCRF indicating insufficient user information.
Option 4: INVITE SIP message for an emergency session without a service URN.
Explanation: The correct trigger is the determination of no IMS-level roaming interfaces coupled with the absence of service information in the AF (Application Function), prompting the P-CSCF to request EPC-level identities from the PCRF to facilitate the emergency registration or session establishment. This ensures that necessary user information is obtained for handling the emergency scenario appropriately, even without prior service information available.