RETURNED TO THE CALLEE. The caller does not ACK within 5 seconds after the callee accepts the call invitation. This is usually a result of a network interruption.
RETURNED TO THE CALLEE. The call invitation received by the callee fails: the call invitation expires 60 seconds since it is sent, if the callee responds to the call invitation but neither the caller nor callee takes any further action ever since (cancel, accept, or decline it).
RETURNED TO THE CALLEE. The caller is offline.