Question: What is indicated by Cause No. 3 in a mobile radio interface Layer 3 specification?
Option 1: The called user's device is out of coverage.
Option 2: The called user's number is not allocated.
Option 3: The network is experiencing congestion.
Option 4: The network through which the call has been routed does not serve the destination desired.
Explanation: Cause No. 3 indicates that the called user cannot be reached because the network through which the call has been routed does not serve the destination desired, highlighting a routing issue rather than a problem with the user's device or number allocation.