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).
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.
United Kingdom
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.
References
- ^ Telcordia GR-506-CORE Issue 3 December 2011, Signaling for Analog Interfaces, Section 17.2.8 Receiver-Off-Hook (ROH) Tone