Question: In a UMTS network, what triggers the increment of the 'Successful US PS Paging Procedures' counter at the SGSN over the Iu interface?
Option 1: Receipt of a 'Location Update Request' message from the MS (Mobile Station)
Option 2: Receipt of a 'Service Request' message with Service Type = Paging Response from the MS
Option 3: Successful completion of a voice call setup over the Iu interface
Option 4: Detection of an MS handover from UMTS to GSM
Explanation: The 'Successful US PS Paging Procedures' counter is incremented when a paging response is received by the SGSN from the MS over the Iu interface, specifically upon receipt of a 'Service Request' message with Service Type = Paging Response from the MS, indicating a successful paging procedure initiated at the SGSN.