Question: In Lawful Interception (LI) for IMS-based VoIP services, what additional identifier(s) must be supported as target identities when interception occurs at the Home Subscriber Server (HSS), if available in subscription data and associable with IMS identities?
Option 1: Only IMEI
Option 2: Only MSISDN
Option 3: Either IMEI or MSISDN, but not both
Option 4: Both IMSI, IMEI, and MSISDN, under specific conditions
Explanation: According to Lawful Interception specifications for IMS-based services, when intercepting at the HSS, multiple identifiers can be used as target identities if certain conditions are met. Specifically, IMSI must be supported if available and associable with IMS identities. IMEI and MSISDN are also supported if the HSS is shared with access services (like PS, EPS) and there's an associable link with IMS identities, ensuring comprehensive coverage for interception while respecting service domain nuances.