The off-hook tone is a telephony signal used to alert a user that the telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 has been left off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

 for an extended period, effectively disabling the telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

. See permanent signal

Permanent signal

Permanent Signal in US telephony jargon, or Permanent Loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time. This is indicated in modern switches by the silent termination after the off hook tone times out and the telephone...

.
In the North American Numbering Plan

North American Numbering Plan

The North American Numbering Plan is an integrated telephone numbering plan administered by Neustar which encompasses 24 countries and territories, including the United States and its territories, Canada, Bermuda, and 16 nations of the Caribbean...

 a quad-frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 tone is used, consisting of frequencies 1400 Hz, 2060 Hz, 2450 Hz, and 2600 hertz, at a cadence

Cadence

Cadence may refer to:Music:*Cadence , a melodic configuration the end of a phrase, section, or piece of music*Cadence Magazine, a monthly review of jazz, blues and improvised music...

 of 0.1s on, 0.1s off . It is played between the dial-tone timeout

Timeout (telecommunication)

In telecommunication and related engineering , the term timeout or time-out has several meanings, including...

 recording ("If you would like to make a call...") and the permanent signal

Permanent signal

Permanent Signal in US telephony jargon, or Permanent Loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time. This is indicated in modern switches by the silent termination after the off hook tone times out and the telephone...

 holding state.