Disconnect supervision

Answer and disconnect supervision are functions in telephony line signaling between a telephone exchange and a connected station. It indicates that the connected call has been answered or is being disconnected, respectively.

For example, the called party may indicate to the telephone exchange that the call is being disconnected by the called party by allowing loop current to flow in the line, or the called party indicates to the exchange that the call is being answered.

For channel-associated signaling (CAS) in Digital Signal 1 (T1) trunks that use E and M signaling (earth & magneto, or ear & mouth signaling), there are only two voice channel states. A channel is idle/on hook when there is no call on it or seized/off-hook/energized by an active call. There is no separate state for answered. It mimics an analog loopstart or groundstart line.

After a channel is initially seized, each device must indicate the progress of a call. 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.

The primary reason for answer and disconnect supervision is for billing. The telephone company and the customer need an accurate accounting of calls. It is standard for telephone companies not to charge for unanswered or unsuccessful calls. All call detail records (CDRs) produced should indicate a call was unanswered or unsuccessful, and therefore, incur no charge from the billing system.

Some systems may not cut through the audio path until there is a positive indication that the called party answered the call—there may not be an audio connection until the answer signal is sent.

...
Wikipedia