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 STIR/SHAKEN attestation level as one of its 26 Gauntlet Engine analysis layers. Calls with C-level or missing attestation 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 attestation levels — like Callro — use this as a structural fraud signal independent of whether the specific number has ever been reported as spam.
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