STIR/SHAKEN Attestation Levels: A, B, C

Every call signed under STIR/SHAKEN carries an attestation level, which is the signing carrier's grade of how sure it is that the caller is allowed to use the number being displayed. There are four outcomes: A, B, C, and blank. That single letter follows your call to the destination carrier and heavily shapes whether the phone rings clean, gets a label, or never rings at all. If you place outbound calls, Call attestation is one of the few STIR/SHAKEN details you can actually influence, so it is worth knowing exactly what each grade means.

The four levels

A, or full attestation, means the call originates on the same carrier as the caller ID number, and that carrier confirms you have the right to use it. B, partial attestation, means the call originates on a different carrier than the one the number belongs to, but the number is registered and valid. C, gateway attestation, means there is no validation of the actual originator, only that the gateway carrier passing the call is valid. Blank, or no attestation, means no STIR/SHAKEN validation was performed at all.

In plain terms, A says the carrier vouches for both the caller and the number, B says it vouches for the number but the caller came from somewhere else, and C says it can only vouch for the gateway. Many terminating carriers treat C and blank calls harshly, sometimes refusing to deliver them to consumer phones at all. Landing at A usually means your numbers live on the same Carrier that signs them, which is part of how a VoIP provider sets up your trunks. This connects to the wider picture in our compliance overview, and it directly feeds why STIR/SHAKEN matters for callers.

A quick way to remember the four is to think of them as shrinking circles of trust:

• A means the carrier knows you and knows the number, and confirms you may use it. This is the level you want on every campaign.
• B means the number checks out but the call came in from another network, so the carrier can vouch for the number and not the caller.
• C means the carrier can only confirm the gateway that handed off the call, with no real knowledge of who started it.
• Blank means the call was never signed at all, which terminating carriers tend to treat as the worst case.

The gap between A and the rest is bigger than it looks. Two campaigns that look identical on paper can see very different delivery if one is signed A and the other lands at C, because the terminating carrier is reacting to the grade, not to your intentions. That is why the level is worth designing your number setup around rather than treating it as something that just happens to your traffic.

How a carrier picks a level

flowchart TD
  A[Call leaves dialer] --> B{Caller ID on same carrier?}
  B -->|Yes and verified| C[Attestation A]
  B -->|No but number valid| D[Attestation B]
  B -->|Originator unknown| E{Gateway valid?}
  E -->|Yes| F[Attestation C]
  E -->|No| G[No attestation]

The signing carrier walks this decision for every call. If your numbers are not registered with the carrier that signs your traffic, you fall straight to C or blank no matter how legitimate your campaign is.

Reaching A is mostly about how your numbers and trunks are provisioned, which is the part we handle. VICIfast configures signing so your traffic earns the attestation it deserves. Check pricing for plans.