 The progress indicators include whether a call is answered or remains unanswered, and when a call is answered, which party disconnects first.These call progress states are important as Telephony systems need to know when the call was attempted, answered, and cleared, hence the term Answer and Disconnect Supervision.Calling party control (CPC), or forward disconnect, is a telecommunication signal sent from a central office to the telephone subscriber's equipment to indicate that the calling party has hung up.