Off-hook tone

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The off-hook tone (also off-hook warning, or howler tone) is a telephony signal for alerting a user that the telephone has been left off-hook without use for an extended period, effectively disabling the telephone line.

North America[]

The off-hook tone in exchanges of the North American Numbering Plan consists of a superposition of tones with the frequencies 1400 Hz, 2060 Hz, 2450 Hz, and 2600 hertz, played at a cadence of 100 ms on and 100 ms off.^[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 an intercept message is announced ("If you would like to make a call...").

A single burst of off-hook tone is sometimes used to indicate to a party that the call is being transferred, notably at 1-800-BELL-SOUTH (800-235-5768).

	Example of a North American Off-hook tone Menu 0:00 Listen to an off-hook tone from North America.
Problems playing this file? See media help.

	High Tone Menu 0:00 Listen to an older 480Hz off-hook tone.
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Some central office switches in the United States, notably older GTD-5 EAX systems, 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 transmitted over a copper POTS circuit; loud enough to be heard across a room from an unused off-hook telephone.

HOWLER[]

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. 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.

See also[]

•  Telephones portal

References[]

External links[]

• High Tone
• Design schematic

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