A signal tone or signalling tone is a steady or pulsating periodic signal typically in the frequency range of sound for indicating a condition, communication protocol state, or serve as an audible warning. It may be composed of multiple frequency components, or could be a pure tone.
In telephone systems, signaling tones are used as call progress tones for in-band indications to subscribers or operators. Certain telephone switching systems used tones, in-band or out-of-band, for signaling on trunks.
Typical well-known call progress tones are dial tone, ringing tone, busy tone, and the 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.
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edit- ^ V. S. Bagad and I. A. Dhotre (2009). Data Communication Systems. Technical Publications. p. 8. ISBN 978-81-8431-698-8.
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