A-Level vs B-Level STIR/SHAKEN Attestation

Every outbound call you place is signed with a STIR/SHAKEN attestation level before it reaches the terminating carrier. That attestation level — A, B, or C — determines how the downstream network treats your call. It influences whether your caller ID displays correctly, whether the call gets flagged as spam, and ultimately whether anyone picks up.

If you operate a predictive dialer or any high-volume outbound operation, attestation level is not an abstract compliance detail. It is the single largest controllable factor in your answer rate. A-level attestation can improve answer rates by 10–20% compared to B-level, because terminating carriers and analytics engines treat A-level calls with higher trust scores.

This guide explains what each attestation level means, why the difference between A and B matters so much, and why the carrier model — not a configuration setting — determines which level you get.

What STIR/SHAKEN attestation levels mean

STIR/SHAKEN (Secure Telephone Identity Revisited / Signature-based Handling of Asserted information using toKENs) is a framework that lets originating carriers cryptographically sign outbound calls. The signature includes an attestation level that tells the terminating carrier how much the originating carrier can verify about the call. For a full technical breakdown, see our STIR/SHAKEN technical deep dive.

A-level attestation (full attestation)

The originating carrier asserts three things:

1. Identity verified — the carrier has authenticated the calling party
2. Number authorized — the carrier has verified the calling party is authorized to use the specific calling number (CID)
3. Direct relationship — the carrier has a direct, verified business relationship with the entity placing the call

A-level is the highest trust signal. It tells every downstream carrier and call analytics platform: “We know exactly who is making this call, and we have confirmed they have the right to use this number.”

B-level attestation (partial attestation)

The originating carrier asserts:

1. Identity verified — the carrier has authenticated the calling party
2. Number authorization unknown — the carrier cannot verify the calling party’s authority over the specific number

B-level means the signing carrier knows its customer but cannot confirm the CID belongs to that customer. This happens structurally when the entity that signs the call (the upstream carrier) is not the same entity that assigned the number to the end user (the reseller).

C-level attestation (gateway attestation)

The originating carrier can only assert:

1. Call origin known — the carrier knows where the call entered its network
2. Identity and number unknown — the carrier cannot verify either the caller’s identity or number authority

C-level is the lowest trust signal. It typically applies to calls entering a carrier’s network from an unknown or unverified source, such as international gateway traffic.

Why the A vs B distinction matters for answer rates

Terminating carriers and call analytics platforms (Hiya, TNS, First Orion, Transaction Network Services) use attestation level as a primary input to their spam scoring algorithms. Here is how the distinction plays out in practice:

A-level calls get favorable treatment. When a call arrives with full attestation, the terminating network knows the originating carrier has done its due diligence. The call is less likely to be flagged, labeled “Spam Risk,” or sent to voicemail. The caller ID displays cleanly. The call rings through.

B-level calls get scrutinized. A B-level attestation tells the terminating network that something is missing — the originating carrier could not confirm number authority. This gap raises a flag. The call may still go through, but it enters the terminating network with a lower trust score. Combined with high call volume, short duration patterns, or other behavioral signals, B-level attestation tips calls into spam territory faster.

The compound effect is significant. Operators who switch from B-level to A-level attestation typically see answer rate improvements of 10–20%. On a campaign dialing 100,000 calls per day, a 15% improvement in answer rates translates to 15,000 additional answered calls — without changing your list, your script, or your dialer configuration.

Real-world impact on caller ID reputation

Caller ID reputation is cumulative. Every call you place contributes to the reputation score associated with your numbers. When those calls carry B-level attestation, each one adds slightly more risk to your number’s reputation profile. Over weeks and months, this compounds:

• Numbers degrade faster under B-level attestation
• “Spam Likely” labels appear sooner on high-volume campaigns
• Number rotation cycles shorten, increasing your CID costs
• You spend more time managing number pools instead of running campaigns

With A-level attestation, your numbers start with higher trust and degrade more slowly. This extends the useful life of each DID and reduces the operational overhead of managing caller ID reputation.

Why resellers deliver B-level attestation

This is not a reseller failing to configure something. It is a structural limitation of the Public Key Infrastructure (PKI) that underpins STIR/SHAKEN. Understanding why requires knowing how the signing chain works.

The PKI signing chain

STIR/SHAKEN uses a certificate-based trust hierarchy:

1. STI-PA (Policy Administrator) — governs the trust framework and authorizes certificate authorities
2. STI-CA (Certificate Authority) — issues SP certificates to authorized carriers
3. SP (Service Provider) — holds the certificate and signs outbound calls

To sign calls with A-level attestation, the SP must:

• Hold its own SP-KI (Service Provider Key Information) certificate
• Have FCC carrier authorization
• Be registered in the Robocall Mitigation Database (RMD)
• Be able to independently verify the caller’s identity AND their authority over the calling number

Where the reseller model breaks

When you buy SIP trunks from a reseller, the call flow looks like this:

Your dialer → Reseller’s platform → Upstream carrier → PSTN

The upstream carrier (Bandwidth, Lumen, etc.) holds the SP-KI certificate. The upstream carrier signs the call. But the upstream carrier’s customer is the reseller — not you. The upstream carrier has verified the reseller’s identity, but it has no direct relationship with you and cannot verify your authority over the DIDs the reseller assigned you.

Result: the upstream carrier signs at B-level. It knows its customer (the reseller). It does not know you.

This is not fixable through configuration. The reseller cannot instruct the upstream carrier to sign at A-level because the upstream carrier is making a factual assertion about what it can verify. It cannot assert number authority for an entity it has never verified. For more detail on this structural limitation, read why reseller VoIP cannot provide A-level attestation.

Can a reseller get its own certificate?

Theoretically, yes. In practice, almost never. Obtaining an SP-KI certificate requires:

• FCC carrier authorization (operating authority filing)
• Registration as a voice service provider
• RMD registration with a robocall mitigation plan
• Acceptance by the STI-PA
• Technical infrastructure to perform real-time call signing

Most resellers are sales and service organizations. They do not have FCC carrier authorization because they are not carriers — they resell another carrier’s network capacity. The barrier is regulatory and structural, not technical.

How SIPNEX delivers A-level attestation

SIPNEX is an FCC-licensed carrier — not a reseller, not a CPaaS wrapper, not a white-labeled trunk from another provider. We hold our own operating authority and our own STIR/SHAKEN Service Provider certificate.

When you place a call through SIPNEX, the signing chain is:

Your dialer → SIPNEX network → STIR/SHAKEN signing (our SP-KI cert) → PSTN

There is no intermediary. SIPNEX is both the carrier that provisions your DIDs and the carrier that signs your calls. Because we have a direct business relationship with you and we assigned the numbers to your account, we can make the full A-level assertion:

1. Identity verified — we authenticated you when you became a SIPNEX customer
2. Number authorized — we provisioned the DIDs to your account; we know they are yours
3. Direct relationship — there is no reseller in between

This is not a premium add-on. Every outbound call through SIPNEX receives A-level attestation by default because the carrier architecture supports it natively.

What this means for dialer operators

For operators running predictive dialers or any high-concurrency outbound platform:

• Higher answer rates — A-level calls are treated with higher trust by terminating carriers
• Longer CID lifespan — numbers degrade more slowly under full attestation
• Cleaner caller ID display — fewer “Spam Likely” or “Scam” labels
• Reduced operational overhead — less time rotating numbers, more time running campaigns
• Compliance alignment — A-level attestation demonstrates good-faith origination practices

How to verify your current attestation level

If you are not sure what attestation level your calls carry, there are several ways to check:

1. Ask your carrier directly

Contact your current SIP trunk provider and ask: “What STIR/SHAKEN attestation level do my outbound calls receive?” If the answer is anything other than a clear “A-level,” or if they hedge with “it depends on the upstream carrier,” you are likely getting B-level.

2. Check the SIP INVITE headers

On a test call, examine the SIP INVITE for the Identity header. This header contains the STIR/SHAKEN PASSporT (Personal Assertion Token). Decode the PASSporT and look at the attest field:

• attest: "A" — full attestation
• attest: "B" — partial attestation
• attest: "C" — gateway attestation

3. Use a verification service

Services like TransNexus or Neustar offer STIR/SHAKEN verification tools that show the attestation level of incoming test calls. Place a call to a test number and check the verification result.

4. Check your carrier’s credentials

Look up your carrier in the FCC’s Robocall Mitigation Database. If they are listed as a “voice service provider” with their own filing, they likely hold their own certificate. If they are not listed, or if they are listed under another carrier’s filing, your calls are being signed by someone else — and likely at B-level.

5. Monitor your answer rates

A sudden or persistent drop in answer rates — especially without changes to your lists or dialing patterns — often indicates an attestation downgrade. Carriers change their STIR/SHAKEN policies without advance notice. What was A-level last month might be B-level today.

Attestation level and SIP trunking

Your SIP trunk provider is the entity that determines your attestation level. Not your dialer software. Not your CRM. Not your compliance vendor. The carrier that signs the call makes the attestation assertion, and that assertion is based on the carrier’s direct knowledge of you and your numbers.

When evaluating SIP trunk providers, attestation level should be a primary selection criterion — not an afterthought buried in a compliance FAQ. Ask every prospective carrier:

1. Do you hold your own STIR/SHAKEN SP-KI certificate?
2. Do you sign calls yourself, or does an upstream carrier sign on your behalf?
3. What attestation level will my outbound calls receive?
4. Can you show me a decoded PASSporT from a test call?

If the answer to question 1 is “no,” or the answer to question 2 is “upstream signs,” the provider cannot guarantee A-level attestation regardless of what their marketing materials claim.

Frequently asked questions

Can I get A-level attestation from a reseller if they use a tier-1 upstream carrier?

No. The tier of the upstream carrier does not change the attestation math. A-level attestation requires the signing carrier to verify your identity and your authority over the calling number. If the signing carrier is the upstream (not the reseller), and the upstream has no direct relationship with you, it will sign at B-level. The reseller’s relationship with a tier-1 carrier does not transfer to you.

Does B-level attestation automatically mean my calls get labeled as spam?

Not automatically, but it significantly increases the risk. B-level attestation is one factor among many that call analytics engines use to score calls. Combined with high call volume, short average call duration, or complaints, B-level tips the scales toward spam labeling faster than A-level would. Think of it as starting every call with a lower trust score — you have less margin before labels appear.

How quickly will I see answer rate improvements after switching to A-level?

Most operators see measurable improvement within the first week. Caller ID reputation is partly real-time (the attestation on the current call) and partly historical (the accumulated reputation of the number). New DIDs provisioned under A-level attestation start clean. Existing numbers that were degraded under B-level will recover over 2–4 weeks as the new A-level attestation rebuilds their reputation profile.

Is there a cost difference between A-level and B-level attestation?

At SIPNEX, A-level attestation is included with every trunk — it is not a premium add-on. Some carriers charge extra for “enhanced STIR/SHAKEN” or “premium attestation,” which usually means they are routing your calls through a different path to get A-level signing. With a direct carrier that holds its own certificate, A-level is the default because the infrastructure supports it natively. Check our pricing page for transparent per-minute rates that include full attestation.

Stop leaving answer rates on the table

If your calls are signed at B-level, you are leaving 10–20% of your answer rates on the table every single day. That is not a compliance nuance — it is a revenue problem with a straightforward fix: move your trunks to a carrier that holds its own STIR/SHAKEN certificate and signs at A-level by default.

SIPNEX is an FCC-licensed carrier with its own SP-KI certificate. Every call we originate is signed at A-level. No intermediaries, no upstream dependencies, no attestation downgrades.

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