Question: What procedure does a Multi-USIM UE initiate to request release to ECM-IDLE state for a USIM due to activity on another USIM?
Option 1: Detach Procedure
Option 2: Service Request procedure (using Extended Service Request message) or a Tracking Area Update procedure
Option 3: Location Update procedure
Option 4: Routing Area Update procedure
Explanation: A Multi-USIM UE initiates the Service Request procedure (using Extended Service Request message) or a Tracking Area Update procedure to request release to ECM-IDLE state for a USIM due to activity on another USIM, as specified in the Connection Release feature. This allows the UE to indicate its request to be released to ECM-IDLE state and also provide Paging Restriction Information if supported by the UE and network.