What Is Attestation Level?
An attestation level is a STIR/SHAKEN grade (A, B, or C) assigned by a carrier to each outgoing call indicating how strongly they have verified that the caller actually owns the phone number being displayed as Caller ID.
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. It is embedded as a cryptographic token in the SIP INVITE header of the call. Carriers, call screening apps, and analytics platforms use attestation levels as a primary spam signal: missing or C-level attestation is strongly correlated with caller ID spoofing.
The Three Attestation Levels
- A — Full Attestation: The originating carrier has verified (1) that the caller is a subscriber on their network and (2) that they are authorized to use the specific number displayed as Caller ID. This is the highest confidence level. Calls from trusted sources — banks, hospitals, government agencies using STIR/SHAKEN-compliant systems — should carry A-level attestation.
- B — Partial Attestation: The carrier has verified the caller's identity as a subscriber on their network, but cannot confirm whether they have the right to use the specific number displayed. The call is from a known source with a potentially unverified number. Common in legitimate business phone systems that route calls through shared number pools.
- C — Gateway Attestation: The call entered the STIR/SHAKEN framework at an intermediate gateway from an external, unknown source. The carrier can only attest to how the call entered their network, not who originated it. This is the default attestation level for calls from offshore VoIP infrastructure and is the strongest structural indicator of potential spoofing.
Using Attestation Levels as a Detection Signal
Callro reads the STIR/SHAKEN attestation level of each incoming call as one of its 26 Gauntlet Engine analysis layers. A-level calls from numbers already in your contact list or related to your carrier's known-good registry are given significantly higher trust scores. C-level calls from unfamiliar numbers with no contact association are given elevated risk scores that feed the overall blocking decision. No single signal is determinative, but attestation level is one of the most reliable structural spam indicators available.
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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