Special information tone

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In telephony, the special information tone is actually a series of three audio tones, which indicate that a telephone call cannot be completed. This series, consisting of a low, medium, and high tone, is used nearly everywhere in the North American Numbering Plan, and is an important type of call progress tone.

The basic SIT protocol is defined by the ITU. The duration of each tone is 330 milliseconds, plus or minus 70, and is within 50 hertz of the reference frequency. The first tone is always the lowest at 950Hz, the middle tone is 1400Hz, and the last is always the highest at 1800Hz.

There are several different but variations of the SIT, with the NANP intercept tone being 913.8Hz for 274ms, then 1370.6Hz for 274ms, and 1776.7Hz for 380ms. Each of these slight variations is within the ITU-specified range.

The first tone is the most important, as this approximately 918Hz signal tells automated dialing equipment that there is a problem. Many predictive dialers, used in telemarketing, pick up on this and delete the telephone number from their database. The Telezapper and other devices use only this tone, so that it is not misunderstood by an actual live person who may be the calling party, as the tone is transmitted every time the called party (or their answering machine) picks up. Using only the single tone without the following two will not work with some dialers, and some now ignore tones completely. Since the invention of these devices, they have both become moot in the United States (and even earlier in several U.S. states) due to no-call lists mandated by law.