Three-beep signal indicating a call did not go through correctly
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.[1][2]
Because the SIT is well known in many countries, callers can understand that their call has failed, even though they do not understand the language of the recorded announcement (e.g., when calling internationally) instead of assuming the recording is voicemail or some other intended function.
Like a dial tone or busy signal, the SIT is an in-band signal intended both to be heard by the caller, and to be detected by automated dialing equipment to determine a call has failed. In North America, the AT&T/Bellcore SIT standard allows the frequency and duration of the tones to vary slightly - making eight distinct messages specifically for automated equipment; indicating not only a failed call, but also the specific reason for the failure (e.g., disconnected number, busy circuits, dialing error, etc.). The equipment can then make an intelligent choice about what to do next. If the circuits were busy, then calling again later makes sense; if the number was disconnected, then calling again is futile.
As an alternative to the in-band SIT tone and recording, SS7 allows call progress advisories to be played/displayed to callers in their own language.
^Feuer, A. (Sep 1981), "Special-Information-Tone Frequency Detection" (PDF), The Bell System Technical Journal, 60 (7), American Telephone and Telegraph Company: 1289–1312, doi:10.1002/j.1538-7305.1981.tb00267.x
^BOC Notes on the LEC Networks - 1994 Signalling - SR-TSV-002275 Issue 2, April, 1994
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