Off-hook tone

[1] The signal is applied to the local loop by the switching system for permanent signal treatment to alert an end user (subscriber) of an off-hook condition of the telephone set, i.e. that the telephone handset should be placed on-hook.

Before playing the signal, a certain timeout has to elapse, and on some systems an intercept message is announced (e.g. "If you'd like to make a call, please hang up and try again.

In either case, the tone is substantially louder than any other signal transmitted over a copper POTS circuit; loud enough to be heard across a room from an unused off-hook telephone.

In the United Kingdom, a warbling signal sounding rather like an alarm siren is played at steadily increasing volume to a telephone left off-hook and unused on telephone lines provided by the BT Group and many PABX extensions.

Telephone lines provided by NTL/Virgin Media tend to use the American-style tones, including a recorded message.