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Disconnect supervision is the feature of some trunk types that allows one device to tell another when it's time to hang up. On some circuits, the PBX removes the battery to let the far end know it should release the trunk. On others, polarity is reversed. Still others use power-denial, which is when the switch removes power for about half a second to the line.
An example of the problem of poor or no disconnect supervision is when an FXO circuit is used with the very common loop-start signaling with non-human systems, such as voice mail. In this scenario, the PBX holds the circuit open until a caller hangs up the phone, except there isn't an actual person here... there's only a voicemail system, which continues recording until it reaches a time-out or the end of its tape, and then hangs up. This is how the long message with no talking is created.
To solve this problem, use a technology with reliable disconnect supervision. For instance, if it's supported, try ground-start signaling instead of loop-start.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
This was first published in May 2003