What is a SIT Tone? The Developer Guide to Stopping Spam
How do you understand special information tones?
A Special Information Tone (SIT) is a standardized three-tone audio sequence defined by ITU-T that telephone networks worldwide use to communicate call status. Autodialers interpret a SIT as a disconnected, invalid, or out of service number — immediately removing it from their 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 INRS?
When Callro identifies an incoming call as spam via its 26-layer gauntlet, Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played. The autodialer's software interprets this as a disconnected number and flags it for removal from the calling database.
This defense exploits a fundamental limitation in how automated calling systems process call results, turning the robocaller's own infrastructure against itself.
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)
- Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played.
- Automatic call rejection with carrier-level disconnect signaling
What is the privacy architecture of Callro?
All call analysis happens entirely on-device. Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played. 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.
Frequently Asked Questions
How do you understand special information tones?
A Special Information Tone (SIT) is a standardized three-tone audio sequence defined by ITU-T that telephone networks worldwide use to communicate call status. Autodialers interpret a SIT as a disconnected, invalid, or out of service number — immediately removing it from their 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 INRS?
When Callro identifies an incoming call as spam via its 26-layer gauntlet, Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played. The autodialer's software interprets this as a disconnected number and flags it for removal from the calling database. This defense exploits a fundamental limitation in how automated calling systems process call results, turning the robocaller's own infrastructure against itself.
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) Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played. Automatic call rejection with carrier-level disconnect signaling
What is the privacy architecture of Callro?
All call analysis happens entirely on-device. Callro rejects the call before it reaches your phone, using Intelligent Network Rejection (INRS) — no audio tone is generated or played. 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.
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