It is standard for telephone companies not to charge for unanswered or unsuccessful calls.
If there was no indication of the call's disconnect, thus no teardown or clearing then all channels in the system would eventually be blocked.
After a channel is initially seized, each device must indicate the progress of a call.
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.
This indicates that answering machines should stop recording, notifies conference call bridges that a participant has left or removes an abandoned call from a hold queue or interactive voice response menu.
The signal may be implemented by removal of battery on the circuit (open switching interval) or by polarity reversal of the tip and ring conductors of the telephone line.
Some digital trunks, such as SS7, use out-of-band signaling to indicate termination of a call.
In modern telephone systems, including voice over IP (VoIP) media gateways, OSI is intentionally used as a supervisory signal to communicate call disconnection.