Question: What cause is returned to a calling mobile station when the called party number indicated by the calling mobile station is no longer assigned?
Option 1: Cause No. 22 "number changed"
Option 2: Cause No. 1 "unassigned (unallocated) number"
Option 3: Cause No. 17 "user busy"
Option 4: Cause No. 34 "no circuit/channel available"
Explanation: When a called party number is no longer assigned, the network returns cause No. 22 "number changed" to the calling mobile station, indicating that the number has been changed or is no longer valid. This allows the calling party to be informed of the change and potentially update their records.