What Is SIT Tone?
A SIT tone (Special Information Tone) is a standardized three-frequency audio sequence that telephone networks use to signal a disconnected or unavailable number — used by Callro to trick robocallers into removing your number from their database.
A SIT tone — Special Information Tone — is a standardized three-frequency audio sequence defined by the ITU-T and the North American telephone network to signal that a called number is disconnected, changed, or otherwise unavailable. When an automated dialing system hears a SIT tone at the start of a call, it interprets the target number as unreachable and flags it for removal from the calling database. Callro generates SIT tones automatically after identifying spam calls, before your phone ever rings.
The Specific Frequencies
The SIT tone is a sequence of three tones: 913.8 Hz, 1370.6 Hz, and 1776.7 Hz, each lasting approximately one-third of a second, followed by a recorded message or silence. Different combinations of high and low variants of these three tones encode different intercept conditions (number changed, disconnected, network congestion, etc.), but all variants trigger automated dialer removal logic.
How Callro Uses SIT Tones
When Callro's Gauntlet Engine identifies an incoming call as spam, it uses Android's ROLE_CALL_SCREENING API to answer the call silently on your behalf — before your phone rings — and plays a SIT tone sequence down the line. The robocall's autodialer receives the signal, interprets your number as disconnected or unavailable, and marks it for removal from its active calling list. Your phone never rings. You never know the call occurred. And the next call from that system is less likely to target you.
For the complete technical breakdown, see What Is SIT Tone? The Sound That Hangs Up On Robocallers.
Frequently Asked Questions
What is a SIT tone?
A SIT tone (Special Information Tone) is a standardized three-frequency audio sequence (913.8 Hz, 1370.6 Hz, 1776.7 Hz) used by telephone networks to signal a disconnected or unavailable number. Automated dialers that detect a SIT tone remove the target number from their calling database.
How does Callro use SIT tones?
When Callro identifies an incoming spam call, it answers the call silently using Android's ROLE_CALL_SCREENING API and plays a SIT tone sequence before your phone ever rings. The robocall autodialer receives the signal, interprets your number as disconnected, and removes it from future calling campaigns.
Do SIT tones stop all robocalls permanently?
SIT tones are effective against automated robocall systems that have removal logic built in — which is the majority of commercial robocall infrastructure. They do not affect human-operated scam calls or systems that ignore SIT signals. Combined with Callro's other 25 detection layers, they significantly reduce recurring call volume.
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