Special information tones

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

Because the SIT is well known in many countries, callers can understand their call 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 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 the call failed, 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, calling back later makes sense; if the number was disconnected, then the calling back is futile. The eight SIT signals are defined below and accompanied by audio files.

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.

A SIT, as defined by the ITU - Telecommunications Standardization Sector (ITU-T), consists of a sequence of three precise tone segments with frequencies of 950 ±50 Hz, 1400 ±50 Hz, and 1800 ±50 Hz, sent in that order.

Each segment is allowed a duration of 330 ±70 ms with a silent interval of up to 30 ms between segments. The nominal tone level is -24 dBm0 (decibels relative to 1 mW measured at the 0 dB TLP) with limits of ±1.5 dB measured with a continuous tone.

...
Wikipedia