Question: What does Cause No. 21 "call rejected" indicate in a mobile radio interface?
Option 1: The equipment sending this cause is busy and cannot accept the call.
Option 2: The equipment sending this cause is incompatible with the calling party's equipment.
Option 3: The equipment sending this cause does not wish to accept the call, despite being able to do so.
Option 4: The equipment sending this cause has experienced a network failure.
Explanation: Cause No. 21 "call rejected" indicates that the equipment sending this cause is intentionally rejecting the call, even though it has the capability to accept it and is not busy or incompatible.