A vs. B vs. C Attestation: What Each Level Means for Your Calls

The Three Levels in Plain Language

The STIR/SHAKEN framework assigns one of three attestation values to every signed call. Each level reflects what the originating carrier actually knows about the call — not what you told them.

A — Full Attestation. The originating carrier authenticated the customer placing the call, verified that the customer is authorized to use the telephone number being presented, and is willing to certify both facts cryptographically. This is the highest signal. Terminating carriers treat A-level calls with the most trust, and some handsets display verified badges when full attestation is present.

B — Partial Attestation. The carrier knows who the customer is and can verify the call's point of origin, but cannot confirm the customer's right to use the specific number being presented. A common scenario: a call center uses a SIP trunk and presents DIDs from a pool that the trunk provider does not directly manage. The carrier knows the call came from a legitimate customer on their network. They cannot vouch for the number.

C — Gateway Attestation. The carrier received this call through an interconnect point — a SIP gateway, a TDM-to-IP conversion, an international hand-off — where the originating party could not be authenticated. No meaningful attestation is possible. C-level calls receive the least trust from downstream analytics.

Why B-Level Is the Silent Revenue Problem

Most call centers operating on standard SIP trunks are getting B-level attestation without knowing it. Their carrier can authenticate the account. But because the DIDs were ported in, purchased from a third-party pool, or assigned through a reseller chain that the carrier does not directly control, the carrier cannot certify number authorization.

B-level calls are not blocked or labeled outright, but they are treated with more scrutiny by spam analytics engines at the major carriers. Those engines score calls based on attestation level combined with behavioral signals. A B-level call that also has a short duration, a high daily call volume, and recent consumer complaints is a candidate for a "Scam Likely" label. A B-level call from a stable number with normal handle times and no complaints typically lands clean. The attestation level tips the balance in edge cases.

Getting to A-level requires choosing a SIP trunking provider that directly controls the numbers in your caller ID pool. UnlimCall issues caller IDs on demand across 33 live markets, with numbers owned and managed within our network — the structural requirement for full attestation. You are not pulling from a pool of numbers that have passed through multiple hands.

Why C-Level Is a Blocking Risk, Not Just a Labeling Risk

C-level attestation reflects a call that entered the US or Canadian network from outside the STIR/SHAKEN ecosystem. Legacy PBX systems with SIP-to-TDM gateways produce C-level traffic. So does any international hand-off that wasn't signed before entering the US network.

The FCC's Robocall Mitigation Database rules allow downstream carriers to block traffic from providers that cannot implement STIR/SHAKEN at all. C-level is different from a provider filing a mitigation program — C-level means the call was signed, but with the lowest possible trust level. In practice, T-Mobile in particular has been aggressive about routing C-level traffic through additional screening that increases post-dial delay and, in some cases, drops calls.

For outbound teams running US and Canadian campaigns, C-level traffic is a structural problem. It is not solved by changing your caller ID rotation frequency or adjusting your dial ratio. It requires changing what enters the SIP network.

What Attestation Level Your Carrier Can Actually Assign

Not every carrier can issue A-level attestation on every call. The Technical Report 69 (TR-69) framework that governs STIR/SHAKEN signing requires the originating provider to have a verified relationship with the number. That means one of three things: they directly issued the DID to you, they ported the number in and manage it as a customer-owned asset, or they have a documented authorization from the number's controlling carrier.

When you shop SIP trunks, the right question to ask is not "do you support STIR/SHAKEN." Every serious US carrier does. The right question is: "For the caller IDs I present, what attestation level will you sign?" If the answer is "B at best," you know why.

See how UnlimCall's on-demand caller ID model compares to the pooled-number approach.

How to Check Your Current Attestation Level

SIP signaling includes the STIR/SHAKEN PASSporT as a JSON Web Token in the Identity header. If you have SIP trace access to your outbound calls, you can inspect the atok field: "A", "B", or "C." Most enterprise dialers and contact center platforms do not surface this in their reporting UI, which is why most teams do not know their attestation level until a customer reports a spam label.

A faster method: call your own cell phone from your outbound number during a fresh campaign session. Check whether the carrier displays the number cleanly, adds "Spam Risk," or shows "Scam Likely." The display behavior is downstream analytics, not raw attestation, but it is a fast proxy for what your traffic looks like to the major carriers.