What is a SIT Tone? The Developer's Guide to Stopping Robocalls

How do you understand special information tones?

A Special Information Tone (SIT) is a standardized sequence of three precisely tuned audio tones defined by ITU-T and used by telephone networks worldwide to communicate call status information. When an autodialer encounters a SIT, it interprets the number as disconnected, invalid, or out of service — and removes it from its call list.

What is the technical specification of a SIT tone?

SIT tones consist of three sequential tones, each lasting approximately 330 milliseconds:

• First tone: 913.8 Hz — Signals an intercept condition
• Second tone: 1370.6 Hz — Specifies the type of intercept
• Third tone: 1776.7 Hz — Confirms the disconnected status

Different combinations of high/low variants for each tone encode different meanings (vacant number, changed number, no-circuit condition, etc.).

How does Callro weaponize SIT tones?

When Callro identifies an incoming call as spam via its 26-layer gauntlet, it silently auto-answers and plays a precisely generated SIT tone sequence. The autodialer's software interprets this as a disconnected number and flags it for removal from the calling database.

This is not a theoretical defense — it exploits a fundamental limitation in how automated calling systems process call results. The robocaller's own infrastructure works against it.

Why does this matter for Android developers?

Android's CallScreeningService API provides the hooks necessary to intercept calls before the ringer fires. Callro extends this with:

• Real-time STIR/SHAKEN attestation checking
• On-device behavioral analysis (no cloud dependency)
• SIT tone playback via AudioTrack for precise frequency generation
• Automatic call rejection with carrier-level disconnect signaling

What is the privacy architecture of Callro?

All SIT tone generation and call analysis happens entirely on-device. Callro never transmits call metadata, contact lists, or audio to any external server. This is an architectural decision, not a policy — there is no server infrastructure to receive the data even if someone tried.