Supervision (telephony)

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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.

E&M signaling[edit]

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.

Lastly, the channel should become free to take new calls when the previous call clears. 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.


This example shows a T1 trunk using E&M wink start signaling only. Other methods can be used, although this was the most common in 20th century private circuits.

Wink start is used to notify the remote side or PBX that it can send the Dialed Number Identification Service (DNIS), also referred to as the Called Number. Automatic Number Identification (ANI) can also be transmitted.

For an incoming call, this occurs:

  1. Calling switch goes off-hook. It sends ABCD bits = 1111.
  2. The called switch sends wink. The ABCD bits transition from 0000 to 1111 for 200 ms, then back to 0000.
  3. The calling switch sees the wink, and then proceeds to send the "DNIS" (called number) information. This is done when inband multifrequency/dual tone multifrequency (MF/DTMF) tones are sent.
  4. The called switch goes off-hook when the call is answered. It sends ABCD bits = 1111.
  5. The audio path is connected, parties can talk, and the billing system registers a call start record.

In an outgoing call the same procedure occurs, but the calling switch and called switch exchange roles.

These occur when a disconnect from the calling party happens:

  1. Calling switch goes on-hook by sending ABCD bits = 0000.
  2. The called switch sees the network go on-hook and the switch goes on-hook. ABCD bits = 0000.
  3. The audio path is disconnected, and the billing system registers a call stop record.

For a disconnect from the called party to the calling party, these steps are reversed.

Supervision signals[edit]

The signal used for supervision varies depending on the type of trunk being used. These signal types include:

  1. Digital E&M - Used on T1 carriers with Channel Associated Signalling trunks
  2. Analog E&M - Used for analog E&M signalling trunks
  3. Analog Disconnect Tone - Used in many South American countries on analog loop start trunks
  4. Analog Open Switching Interval (OSI) - Used in North America on analog loop start trunks[1]
  5. Digital ISDN - Used with both PRI and BRI digital trunks

See also[edit]


As described above, when the line is in an idle condition or "on-hook" there is no presence of dial tone. If in fact a fast busy tone is heard, then there is something wrong with the line itself.

Some of these signals have the Busy signal sound within them.