What Is Attestation Level?
Attestation levels (A, B, C) are STIR/SHAKEN cryptographic grades carriers assign to indicate how verified a caller's identity and number ownership are.
An attestation level is the grade assigned to an outgoing call by the originating carrier under the STIR/SHAKEN framework, indicating how strongly the carrier has verified the caller's right to use the number displayed as Caller ID. Embedded as a cryptographic token in the SIP INVITE header, attestation levels serve as a critical defense signal against caller ID spoofing.
The Three Attestation Levels
- A — Full Attestation: The originating carrier has verified that the caller is a subscriber on their network and is authorized to use the specific number displayed. Calls from A-attested numbers represent verified identities.
- B — Partial Attestation: The carrier verified the caller's account but cannot confirm whether they own the specific number shown. Common in legitimate PBX business phone pools.
- C — Gateway Attestation: The call entered the network at an intermediate gateway from an unknown external source. C-level attestation is typical of offshore VoIP routes and represents a primary robocall signal.
Using Attestation Levels as a Detection Signal
Callro reads the carrier's STIR/SHAKEN verification status (Passed, Failed, or Unverified) as reported by Android as one of its Gauntlet Engine analysis layers. Calls with unverified status from unrecognized originators are subjected to deeper heuristic scoring. Read more in our analysis of the FCC 2026 STIR/SHAKEN framework and neighbor spoofing mechanics.
Frequently Asked Questions
What is a STIR/SHAKEN attestation level?
An attestation level is a grade (A, B, or C) assigned by the originating carrier to each call under the STIR/SHAKEN framework. It indicates how strongly the carrier has verified that the caller owns the specific number displayed as Caller ID. A is full verification, B is partial, C is gateway (unknown origin).
What does C-level attestation mean?
C-level (Gateway) attestation means the call entered the STIR/SHAKEN framework from an unknown external source at a network gateway. The originating carrier can only confirm how the call arrived at their network, not who made it or whether the displayed Caller ID is authentic. C-level attestation is a strong indicator of potential caller ID spoofing.
How does attestation level help stop robocalls?
Because caller ID spoofing cannot generate A-level STIR/SHAKEN attestation (which requires the carrier to verify the caller owns the displayed number), spoofed calls typically carry B-level, C-level, or no attestation. Apps that read carrier verification status — like Callro — use this as a structural signal independent of whether the specific number has ever been reported as spam.
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