The reorder tone, also known as the fast busy tone, or the congestion tone, or all trunks busy (ATB) tone is an audible call progress tone in the

public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...

(PSTN) that is returned to a calling party to indicate that the call cannot be processed through the network. The tone characteristics vary by country or telephone administrations. In North America it is a dual-

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

tone of 620 Hz and 480 Hz interrupted 120 times per minute at a cadence of 0.25

second The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ea ...

s on, 0.25 off, i.e., two beeps per second. In EU countries and those following ETSI (European Telecommunication Standards Institute) recommendations, the cadence is the same as North America, i.e. 0.25s on / 0.25s off, but with a 425 Hz tone. The UK reorder tone uses a 400 Hz tone with a cadence of 0.4 seconds on, 0.35 seconds off, 0.225 seconds on, 0.525 seconds off. In some instances the Number Unobtainable Tone may be used instead. This is a continuous, uninterrupted 400 Hz tone. The signal is used to indicate that the destination is unreachable, either because all circuits (trunks) are busy, the called number is out of service, the call is unroutable, or sometimes that an invalid code has been dialed. A PBX also often indicates an invalid

extension Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ext ...

, while dialing an invalid

telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...

on the PSTN usually results in playing the triple

special information tone In telephony, a special information tone (SIT) is an in-band international standard call progress tone consisting of three rising tones indicating a call has failed. It usually precedes a recorded announcement describing the problem.BOC Notes o ...

and a recorded announcement. Some European networks may return the reorder tone as a generic error signal for invalid or incorrect numbers or where a particular call type is not supported or has been barred. Some switches, notably in Europe where open dial plans are used, analyse a number as it's dialled may even return a reorder tone before the complete number is dialled if the initial sequence of digits are invalid. Many telephone companies in North America and in the United Kingdom play the reorder tone after the reading of a recorded announcement explaining the reason of call failure. The reorder tone is sometimes confused with the

busy signal Reanno Devon Gordon (born 24 January 1979), better known by his stage name Busy Signal, is a Jamaican dancehall reggae artist. Biography Reanno Devon Gordon p/k/a Busy Signal was born in Saint Ann Parish,Johnson, Carolyn (2006)A Very Busy Chi ...

because they use the same frequencies and differ only in that the busy signal's beeping sounds have a slower cadence than those of the reorder tone.

References

Telephony signals {{telephony-stub