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Uploaded on Aug 3, 2010
The off-hook tone (also receiver off-hook tone or off-hook warning) is a telephony signal used to alert a user that the telephone has been accidentally left off-hook for an extended period, effectively disabling the telephone line.
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 off-hook telephone.
Receiver Off-Hook Tone is 1400 Hz, 2060 Hz, 2450 Hz and 2600 Hz at 0 dBm0/frequency on and off every .1 second. On some older space division switching systems Receiver Off-Hook was 1400 Hz, 2060 Hz, 2450 Hz and 2600 Hz at +5 VU on and off every .1 second. On a No. 5 ESS this continues for 30 seconds. On a No. 2/2B ESS this continues for 40 seconds. On some other AT&T switches there are two iterations of 50 seconds each.