Question: What triggers the Home Location Register (HLR) to send a 'Forward Check SS Indication' request message to the Visitor Location Register (VLR), in relation to the 'Check SS' restoration indicator?
Option 1: Receipt of an 'Update Location' request from the VLR when the 'Check SS' indicator is set to 'Check Not Required'.
Option 2: Successful completion of a subscriber's call setup procedure.
Option 3: Periodic location update timers expiring in the HLR.
Option 4: Receipt of an 'Update Location' request from the VLR when the 'Check SS' indicator is set to 'Check Required'.
Explanation: The correct answer highlights a scenario where the HLR's action is tied to the state of the 'Check SS' indicator and the receipt of an 'Update Location' request, demonstrating understanding of how restoration indicators in the HLR interact with location update procedures between the HLR and VLR.