Attestation level — STIR/SHAKEN attestation tiers
What is Attestation level?
Attestation level is the three-letter grade (A, B, or C) the originating carrier stamps onto every outbound STIR/SHAKEN call to declare how thoroughly it verified the caller. A (full attestation) means the carrier authenticated the caller and confirmed its right to use the caller-ID it presented. B (partial attestation) means the carrier knows who the caller is but cannot confirm it owns the caller-ID. C (gateway attestation) means the call entered the network from a source the carrier cannot verify, so only the gateway path is known.
More detail
The originating carrier's verification depth, not the terminating one, decides the level. Every outgoing call carries a signed PASSporT header with the attestation tier, and the terminating carrier weighs that tier along with its own spam analytics when picking down or flagging the delivered call.
Because A-tier attestation is what the terminating carrier expects from a business shipper's pool, a sender that can prove it owns its caller-ID normally sticky-ships A rather than leaving the default gateway-level (C) stamp. Answer-rate differences between A-tier and C-tier pools are the everyday signal that the tier matters.
Frequently asked
- What's the practical difference between A and B attestation?
- A-level (full) means the carrier verified both the caller's identity and its right to use the caller-ID; B-level (partial) verifies the caller's identity but not the number's ownership. Terminating carriers usually treat both politely but only A earns the cleanest answer-rate boost.
- Why does C-level attestation matter to a CPaaS buyer?
- C-level (gateway) applies where the originating carrier only knows the point of entry, not the caller's identity. In practice it's what voice gateways ship by default, and as soon as the recipient's analytics see a gatewayed mix, its confidence drops and the landed answer-rate drops with it.
See also
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