Question: What is the primary reason for using Cause No. 18 "no user responding" in mobile radio interface Layer 3 specification?
Option 1: The user has rejected the call.
Option 2: The network is experiencing congestion.
Option 3: The user does not respond to a call establishment message within the prescribed time period.
Option 4: The user's device is out of coverage.
Explanation: Cause No. 18 "no user responding" is used when a user fails to respond to a call establishment message with either an alerting or connect indication within the allocated time period, which is defined by the expiry of timers T303 or T310.