Special information tones

In telephony, a special information tone (SIT) is an international standard three beep signal indicating a call did not go through and usually precedes a recorded announcement explaining the problem.^[1] As the SIT is well known in many countries, callers who do not speak the language of the announcement will still understand that the call has failed.

Additionally, there are eight variations of the SIT signal to indicate why the call failed (e.g. disconnected number, busy circuits, dialing error, etc). All eight SIT signals are defined below with recorded examples.

The SIT was developed because automated dialing equipment, modems, and network call detector/classifiers simply cannot decipher between a live answer or a recording, or what is said in a recorded announcement.

International Telecommunication Union definition

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.^[1][2]

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.

The difference in level between any two segments is required to be less than 3 dB.

The above requirements apply at the point at which tones are applied to the network.

AT&T/Bellcore Standard SIT composition

AT&T further refined the above ITU standard to include the following variants to convey condition encountered. This enables telephone equipment to determine and log the reason a 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. The third tone segment may be of long or short duration but is limited to the lower frequency state. Currently, the third tone segment has been assigned both a fixed long duration and a fixed lower frequency. This fixed assignment of the third tone provides a reference or calibration point for detection devices.^[1]

Segment durations

• Short duration = 276 ms
• Long duration = 380 ms

Frequencies for use in SITs

First segment	Second segment	Third segment
(high) 985.2 Hz	(high) 1428.5 Hz	(low) 1776.7 Hz
(low) 913.8 Hz	(low) 1370.6 Hz

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.

SIT example recordings and encoding scheme ^[1]

Name	Code	Duration	Frequency	Description	Example
Reorder – intraLATA	RO'	short, long, long	low, high, low	Incomplete digits, internal office or feature failure – local office
Vacant Code	VC	long, short, long	high, low, low	Unassigned N11 code, CLASS code or prefix
No Circuit – intraLATA	NC'	long, long, long	high, high, low	All circuits busy – local office
Intercept	IC	short, short, long	low, low, low	Number changed or disconnected
Reorder – interLATA	RO' '	short, long, long	high, low, low	Call failure, no wink or partial digits received – distant office
No Circuit – interLATA	NC' '	long, long, long	low, low, low	All circuits busy – distant office
Ineffective/Other	IO	long, short, long	low, high, low	General misdialing, coin deposit required or other failure
Future Use	–	short, short, long	high, high, low	Reserved for future use.

Other uses

Because many predictive dialers (used in telemarketing) respond to SITs, consumer devices such as the Telezapper play an Intercept SIT to trick the telemarketer's equipment into flagging a called number as disconnected.

Additionally, the above recordings of SITs could be used on a voicemail or answering machine to achieve a similar effect.

Some telephone companies advise against playing false SITs on active lines and/or message equipment as it could cause caller confusion - especially in an emergency.

See also

• Telephone
• Pat Fleet The voice behind most intercept recordings.

References

1. ^ ^{a b c d} BOC Notes on the LEC Networks - 1994 Signalling - SR-TSV-002275 Issue 2, April, 1994
2. ^ International Telecommunication Union (1998)

• International Telecommunication Union (March 1998), "Special information tone", Technical characteristics of tones for the telephone service, E: Overall Network Operation, Telephone Service, Service Operation and Human Factors / Q: Switching and Signalling, ITU-T Rec. E.180/Q.35, http://www.itu.int/itu-t/recommendations/index.aspx?ser=E 

External links

• World PSTN Tone Database - Special information tones
• Call Progress Tones (Cisco Systems)
• artofhacking.com's database of SIT tones for the US and other countries