Question: What response does a Mobility Management Entity (MME) send to a Visitor Location Register (VLR) when receiving an SGsAP-ALERT-REQUEST for an unknown IMSI (International Mobile Subscriber Identity)?
Option 1: SGsAP-ALERT-ACCEPT with a request for additional information
Option 2: SGsAP-ALERT-REJECT with 'IMSI unknown' as the SGs cause information element value
Option 3: SGsAP-LOCATION-UPDATE-REQUEST to retrieve more subscriber data
Option 4: SGsAP-PAGING-REQUEST to locate the UE (User Equipment)
Explanation: When an MME receives an alert request for an IMSI it doesn't recognize, it must notify the VLR of this failure. The appropriate response in this scenario, as per telecom protocols for handling unknown subscriber identities, is to reject the alert with a clear indication of the issue (i.e., 'IMSI unknown'). This ensures the VLR understands the reason for the rejection and can proceed accordingly, maintaining efficient network operation.