Ordinary analog line signaling protocol does not have any specific signal returned to the terminating side when the originating side hangs up the call.. The terminating side can only "guess" the call status by listening to the audio signal returned from other side. If after sending the path confirmation signal, and there is no response from the originating side within a certain length of time, Abacus can safely say that the originating side has hung up. So it can start to release the call. This is in essence emulating a real telephone call used by human.