Disconnect supervision means the CO has the ability to release a trunk when the party at the CO disconnects and the system is able to recognize the release signal. In general, a CO in the United States provides disconnect supervision for incoming calls but not for outgoing calls. Many other countries do not provide disconnect supervision for either incoming or outgoing calls.

The system must provide the assurance that at least one party on the call can control dropping the call. This avoids locking up circuits on a call where no party is able to send a disconnect signal to the system. Internal operations must check to ensure that one party can provide disconnect supervision. An incoming trunk that does not provide disconnect supervision cannot terminate an outgoing trunk that does not provide disconnect supervision.

In a DCS environment an incoming trunk without disconnect supervision can terminate to an outgoing DCS trunk connecting two nodes. The incoming trunk is restricted from being transferred to a party without disconnect supervision on the terminating node. This is because through messaging the terminating node knows that the originating node cannot provide disconnect supervision. This messaging is not possible with non-DCS tie trunks, and the direct call is denied.

Administration is provided for each trunk group to indicate whether it provides disconnect supervision for incoming calls and for outgoing calls.