Question: Under what condition, related to UE's reachability, may an MME invoke the Service Request procedure directly after receiving an SGsAP-PAGING-REQUEST message from the VLR in Deployment Option 3?
Option 1: When the UE is in a high-priority EMM (EPS Mobility Management) mode.
Option 2: If the UE supports only Circuit Switched (CS) services.
Option 3: When the UE is temporarily unreachable for paging due to using eDRX (extended Discontinuous Reception) and an 'SMS indicator' is set in the Service indicator information element.
Option 4: Upon detection of a voice call request from the UE.
Explanation: This option is correct because, in Deployment Option 3, if the MME detects that the UE is temporarily unreachable for paging due to using eDRX and the Service indicator information element indicates an 'SMS indicator', it may invoke the Service Request procedure.