Question: What triggers the increment of the 'Number of unknown IMEI answers' measurement in a GSM network?
Option 1: Receipt of a valid IMEI from a newly attached device
Option 2: Receipt of 'MAP_ECK_IMEI' service confirmation with a 'user error' parameter referring to unknown equipment
Option 3: Successful completion of a CALL SETUP service for any MS
Option 4: Periodic synchronization between the MSC and EIR
Explanation: The correct answer is based on how unknown IMEI answers are counted. When the network receives a 'MAP_ECK_IMEI' service confirmation with an error indicating the equipment (identified by its IMEI) is unknown, this event increments the measurement. This occurs when a device with an unrecognized IMEI attempts to connect, prompting the Equipment Identity Register (EIR) to respond with such an error, crucial for network security and managing unauthorized or stolen devices.