{{Short description|Audible alert on a telephone left off-hook}} {{refimprove|date=January 2008}}

The '''off-hook tone''' (also '''off-hook warning''', '''howling tone,''' 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&nbsp;Hz, 2060&nbsp;Hz, 2450&nbsp;Hz, and 2600&nbsp;Hz, played at a cadence of {{val|100|u=ms}} on and {{val|100|u=ms}} off.<ref>[http://telecom-info.telcordia.com/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-506& Telcordia GR-506-CORE Issue 3 December 2011, ''Signaling for Analog Interfaces''], Section 17.2.8 ''Receiver-Off-Hook (ROH) Tone''</ref> 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. If you need help, hang up and then dial your operator. This is a recording.").

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).

{{listen |filename=Offhook Tone.ogg |title=Example of a North American Off-hook tone |description=Listen to an off-hook tone from North America. }}

{{listen |filename=Tone_480Hz.ogg |title=High Tone |description=Listen to an older 480Hz off-hook tone. }}

Some central office switches in the United States, notably older GTD-5 EAX systems, utilize a single frequency tone, 480&nbsp;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== * {{portal-inline|Telephones}}

==References== {{reflist}}

==External links== *[http://www.tech-faq.com/frequencies-of-the-telephone-tones.html High Tone] *[http://pe2bz.philpem.me.uk/Misc/-%20Analog/-%20Osc/TELEPHONE/H-325-OffHookToneGenerator/offhook.html Design schematic]

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Category:Telephony signals