Call attestation A, B, and C explained

Attestation is the level of confidence a carrier attaches to your caller ID when it signs a call. There are three levels — A, B, and C — and the higher the level, the more the carrier is vouching for you, which is exactly what keeps your number off the Spam Likely list.

Call attestation is part of STIR/SHAKEN, the system carriers use to cryptographically sign caller ID. When your call is signed, the originating carrier stamps it with one of these three grades, and every carrier the call passes through can read that grade.

The three levels

Each level answers two questions: does the carrier know who you are, and do they know you are allowed to use this number?

• A (full attestation): the carrier knows the customer and confirms they have the right to use the Caller ID spoofing-free number on the call. This is the level you want.
• B (partial attestation): the carrier knows the customer but cannot confirm the number's provenance. The call is authenticated, but the number is not fully verified.
• C (gateway attestation): the carrier is just passing the call through and can vouch for almost nothing about it.

flowchart TD
  Q1{Carrier knows customer?}
  Q1 -- No --> C[Attestation C: pass-through]
  Q1 -- Yes --> Q2{Right to use this number?}
  Q2 -- No --> B[Attestation B: partial]
  Q2 -- Yes --> A[Attestation A: full]
  A --> R[Lowest Spam Likely risk]
  B --> M[Medium risk]
  C --> H[Highest risk]

Why the level matters

Downstream carriers and call-analytics services use attestation as one input when they decide whether to slap a Spam Likely label label on your call. A clean attestation A call is treated as trustworthy. A call signed at C carries almost no weight and is far more likely to get flagged, because gateway-level signing tells the receiving carrier nothing useful.

Attestation is not the only signal — call volume and complaint rates matter too — but it is the one you have the most direct control over. Getting to A is mostly about your carrier relationship and the numbers you dial from.

How to reach attestation A

You earn A by dialing from a DID (direct inward dialing) that your carrier knows you own. That usually means:

1. Buy your numbers directly from the Carrier that signs your calls, so they have a record that you own them.
2. Set the campaign caller ID to one of those owned numbers, not a borrowed or unverified one.
3. Keep the STIR/SHAKEN integration enabled so VICIdial hands the right identity data to the carrier before each call.

How to check what level you are getting

Attestation is decided at the signing carrier, so you cannot read it off VICIdial directly. The practical way to check is to ask your carrier what level your calls are signing at, since they have the record. Some carriers expose it in a portal or on call detail records; others will tell you only if you ask support.

A few things that quietly drop you below A even when you think you have it right:

• The campaign caller ID does not match a number registered to your carrier account.
• A list or lead DID (direct inward dialing) override is sending a different number than you expect.
• The STIR/SHAKEN integration is misconfigured, so the carrier signs at C by default.

The mechanics of getting calls signed in the first place are covered in our STIR/SHAKEN overview, and attestation fits into the bigger carrier picture in the carrier integration guide.

Good attestation starts with a stable, known-good server identity. A VICIfast box ships with a fixed public IP and a branded subdomain in under 40 seconds, which gives your carrier a clean, consistent origin to vouch for — see pricing to spin one up.