Special information tone

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.).

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.

[1] By using slight variations of tone frequencies and durations, the AT&T SIT conveys to automated equipment why the call failed.

In the Bellcore specification the first and second tone segments vary based on the condition encountered: having either a short or long duration, and either a lower or higher part of the frequency band allowed by the ITU-T.

[1][2] The interval between the segments of SITs is between 0 and 4 ms. To minimize the number of callers who may abandon the call without listening to the announcement, the nominal time gap between the third tone segment and the beginning of the announcement is set as close to zero as possible, with an allowed maximum of 100 ms. [2] The recordings below play each SIT sequence two times.