Tone out Robo Calls

Special Information Tones (SIT) are used to notify callers that their call has failed (ie; the number has changed or has been disconnected) These special information tones are in sequences of three precise tone segments with frequencies of 950 ±50 Hz, 1400 ±50 Hz, and 1800 ±50 Hz, sent in that order. Automated calling systems typically will end the call once the SIT tone commences.