In some regions of the world, the Central Office (CO) sends a busy tone for the disconnect message. With Busy Tone Disconnect (BTD), the switch disconnects analog loop-start CO trunks when the CO sends a busy tone.

A call that originates from or terminates to a telephone that uses a BTD-enabled trunk has a Call Classifier port connected to the trunk. The Call Classifier port connects after the call is answered and stays connected on the trunk until the station hangs up, or a BTD signal is received from the CO. If only one BTD trunk is on a call when the BTD signal is received, the call is dropped. If the call is a conference call, only the trunk is dropped. The rest of the parties stay connected.