The off-hook tone (also receiver off-hook tone, off-hook warning, or howler tone) is a telephony signal used to alert a user that the telephone has been left off-hook without use for an extended period, effectively disabling the telephone line. See permanent signal.
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North America
In the North American Numbering Plan a quad-frequency tone is used, consisting of frequencies 1400 Hz, 2060 Hz, 2450 Hz, and 2600 hertz, at a cadence of 0.1s on, 0.1s off. It is played between the dial-tone timeout recording ("If you would like to make a call...") and the permanent signal holding state. A single burst of off-hook tone is sometimes used to indicate to a party that their call is being transferred, notably at 1-800-BELL-SOUTH (800-235-5768).
Some US CO switches, notably older GTE GTD-5s, utilize a single frequency tone, 480 Hz, known as "High Tone" for this purpose. In either case, the tone is substantially louder than any other signal which travels over a copper POTS circuit; loud enough to be heard across a room from an unused off-hook telephone.
United Kingdom
In the United Kingdom, a warbling signal sounding rather like an alarm siren is played (at steadily increasing volume) down a telephone left off-hook and unused on telephone lines provided by BT and many PABX extensions. It is sometimes referred to as a "howler."
In some cases it is composed of the DTMF tones * and # played alternately.
Telephone lines provided by NTL/Virgin Media tend to use the American style tones, including a recorded message.