Question: What triggers an MME to send an Alert MME Notification message on the S3 interface to the associated SGSN?
Option 1: When a UE initiates a voice call.
Option 2: When ISR is deactivated.
Option 3: When the MME receives a notification from the HSS.
Option 4: When ISR is activated, except under specific conditions.
Explanation: According to the specification, an Alert MME Notification message is sent by the MME to the associated SGSN on the S3 interface when ISR (Idle mode Signaling Reduction) is activated, except under certain conditions specified in 3GPP TS 23.272. This allows the MME to request notification when any activity from the UE is detected.