Special information tone
In telephony, special information tones are a series of three audio tones, which indicate that a telephone call cannot be completed and is typically followed by a recorded announcement explaining the reason for the call failure.
In addition to indicating a general call failure, SIT's permit automated dialing equipment, modems, and network call detector/classifiers a way to automatically classify WHY the call failed.
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 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
Frequencies for use in SITs
| First Segment | Second Segment | Third Segment |
| (low) 913.8 Hz | (low) 1370.6 Hz | (low) 1776.7 Hz |
| (high) 985.2 Hz | (high) 1428.5 Hz |
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 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 and delete the dialed telephone number from their database upon encountering one, consumer devices such as the Telezapper and others have been developed which transmit a false Intercept SIT every time the called party (or their answering machine) answer.
As a result, many dialers now ignore the tones completely. While the implementation of state and Federal "do not call" laws in the United States were intended to render such devices moot, recent innovations in telemarketing, such as systems which ignore the SIT signalling, have brought about a renewed interest in the use of the tones to foil telemarketers. Audio files of the tones are available on several web sites, which allow individuals to record them onto their answering devices for playback to unwanted callers.
Reference
External links
- artofhacking.com's database of SIT tones for the US and other countries
- This Is a Recording - Disconnected / No Longer in Service
