Special information tones

In telephony, a special information tone is the three beep signal heard at the beginning of telephone company recorded announcements. There are eight variations of the SIT signal, all with different meanings. While all versions of the SIT signal indicate that a telephone call failed, the different variations of the SIT indicate WHY the call failed (e.g. disconnected number, busy circuits, dialing error, etc - see below for complete list).

The SIT was developed because automated dialing equipment, modems, and network call detector/classifiers simply cannot decipher what is said in a recorded announcement. Also, because the SIT is an international standard, telephone equipment dialing numbers in another country can still determine the reason a call failed - even though operators may not understand the language of a recorded announcement. All eight SIT signals are defined below with recorded examples.

International Telecommunication Union definition. ^[1]

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.

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.

SIT composition^[1]

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.

Segment durations

• Short Duration = 276 ms
• Long Duration = 380 ms

Frequencies for use in SITs

The interval between the segments of SITs is between 0 and 4 ms. To minimize the 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	RO' SIT Reorder - intraLATA (2x) Problems listening to the file? See media help.
Vacant Code	VC	Long, Short, Long	high, low, low	Unassigned N11 code, CLASS code or prefix	VC SIT Vacant Code (2x) Problems listening to the file? See media help.
No Circuit - intraLATA	NC'	Long, Long, Long	high, high, low	All circuits busy - local office	NC' SIT No Circuit - intraLATA (2x) Problems listening to the file? See media help.
Intercept	IC	Short, Short, Long	low, low, low	Number changed or disconnected	IC SIT Intercept (2x) Problems listening to the file? See media help.
Reorder - interLATA	RO' '	Short, Long, Long	high, low, low	Call failure, no wink or partial digits received - distant office	RO' ' SIT Reorder - interLATA (2x) Problems listening to the file? See media help.
No Circuit - interLATA	NC' '	Long, Long, Long	low, low, low	All circuits busy - distant office	NC' ' SIT No Circuit - interLATA (2x) Problems listening to the file? See media help.
Ineffective/Other	IO	Long, Short, Long	low, high, low	General misdialing, coin deposit required or other failure	IO SIT Ineffective/Other (2x) Problems listening to the file? See media help.
Future Use	--	Short, Short, Long	high, high, low	Reserved for future use.	Future SIT Future Use (Not Used) (2x) Problems listening to the file? See media help.

Other uses

Because many predictive dialers (used in telemarketing) respond to SITs, consumer devices such as the Telezapper have been developed which play an Intercept SIT in order to trick the telemarketer's equipment into flagging a called number as disconnected. (See Telezapper for more information.)

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

Note that 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.

References

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
• This Is a Recording - Disconnected / No Longer in Service

See also

• Telephone
• telemarketing