High Quality Content by WIKIPEDIA articles! 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).

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