# Signal tone

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A **signal tone** or **signalling tone** is a steady or pulsating [periodic](/source/Periodic_function) signal typically in the frequency range of [sound](/source/Sound) for indicating a condition, [communication protocol](/source/Communication_protocol) state, or serve as an audible warning. It may be composed of multiple frequency components, or could be a [pure tone](/source/Pure_tone).

In telephone systems, signaling tones are used as [call progress tones](/source/Call_progress_tone) for [in-band](/source/In-band) indications to subscribers or operators. Certain telephone switching systems used tones, in-band or out-of-band, for [signaling](/source/Signaling_(telecommunications)) on trunks.

Typical well-known call progress tones are [dial tone](/source/Dial_tone), [ringing tone](/source/Ringing_tone), [busy tone](/source/Busy_tone), and the [reorder tone](/source/Reorder_tone).[1] A loud stutter tone is used to alert subscribers of a handset left off-hook, effectively disabling the circuit for receiving calls.

Telephone service subscribers may subscribe to services, such as call forwarding, which may indicate function by a stutter dial tone.

## See also

- [Musical tone](/source/Musical_tone)
- [Dual-tone multi-frequency signaling](/source/Dual-tone_multi-frequency_signaling) (DTMF)
- [Signaling (telecommunications)](/source/Signaling_(telecommunications))

## References

1. V. S. Bagad and I. A. Dhotre (2009). [*Data Communication Systems*](https://books.google.com/books?id=srkNoDo3mbwC&dq=%22signalling+tone%22&pg=SA8-PA8). Technical Publications. p. 8. ISBN 978-81-8431-698-8.

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Adapted from the Wikipedia article [Signal tone](https://en.wikipedia.org/wiki/Signal_tone) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Signal_tone?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.