Question: What is the primary reason for sending Cause #22 – Congestion to the UE?
Option 1: The MSC is temporarily not reachable via the EPS part of the network.
Option 2: There is a failure in the ESM message contained in the EMM message.
Option 3: The CS domain is not available and SMS in MME is not supported.
Option 4: The network is experiencing congestion, resulting in no channel or facility being busy/congested.
Explanation: Cause #22 – Congestion is sent to the UE because of congestion in the network, such as no channel or facility being busy/congested. This indicates that the primary reason for sending this cause is due to network congestion, making option 4 the correct answer.