busy tone (BT)

A busy tone is an acoustic signal used in telephone networks and mobile networks that signals to the caller that no connection can be established with the called party because the called party is either currently on the phone or does not want to accept the call.

The busy signal is an audible tone of 425 Hz that is repeated in short succession. The tone duration and pause duration together have a length of 960 ms, the busy tone lasts 480 ms and the pause duration also 480 ms. Previously, the tone duration was 125 ms and the pause duration was 475 ms. The busy tone merely indicates to the caller that the called subscriber is not available. The busy tone does not indicate whether the called subscriber is currently conducting a call or whether the subscriber line cannot be reached for other reasons.