Call Attestation Levels (A, B, C) Explained

What attestation levels mean

STIR/SHAKEN gives the originating carrier three options when signing a call. The level reflects two things: whether the carrier knows the customer placing the call, and whether the customer is authorized to use the specific number that is appearing as the caller ID.

• A-level (Full): the carrier has verified both the customer's identity and the customer's right to use the calling number. Strongest signal; best for answer rates.
• B-level (Partial): the carrier has verified the customer but cannot fully verify the right to use this specific number. Common for ported numbers and verified caller ID arrangements.
• C-level (Gateway): the carrier originated or transited the call but cannot verify the customer or the number. Often associated with international gateways or wholesale interconnect traffic.

A-level: full attestation

A-level is the level you want for any call where answer rates matter. It signals to the receiving network that the originating carrier has done the underlying verification work: the customer is known, billing relationships exist, and the right to use the displayed number has been confirmed.

• Granted by carriers that provision and own the number directly.
• Standard for VeraDial-purchased US and Canadian business numbers.
• Strongest defense against 'Spam Likely' or 'Scam Likely' carrier labels.

B-level: partial attestation

B-level applies when the carrier has verified the calling business but cannot fully confirm authorization for the specific number. This is common when a business uses a verified caller ID that originates from a different provider, or when number ownership records are incomplete.

• Common for ported numbers in transition.
• Used for verified caller ID — outbound calls placed from a number you already own and have verified with your provider.
• Better than C-level or unsigned, but typically less effective than A-level for answer rates.

C-level: gateway attestation

C-level says the carrier saw the call and is willing to identify itself, but cannot vouch for the customer or the number. This commonly applies to calls originated outside the country or routed through wholesale carriers without a direct relationship to the calling business.

• Frequently associated with international or wholesale traffic.
• Treated as low trust by receiving carriers.
• Often filtered or labeled as suspicious on receiving phones.

How to find out what level your calls get

Most receiving phones do not show the attestation level directly — they show a label such as 'Verified' or 'Spam Likely.' The most reliable way to know your level is to ask your provider in plain terms; a provider that signs at A-level usually says so explicitly.

• Ask your phone provider what attestation level they sign your outbound calls with.
• Test calls to phones on different networks (AT&T, Verizon, T-Mobile, Rogers, Bell, Telus) and note which ones display 'Verified' or similar labels.
• Look for STIR/SHAKEN attestation language in your provider's product documentation.
• Avoid services that obscure or do not disclose attestation behavior.

Why the level matters for small business

Attestation is an input to the receiving carrier's spam filtering decisions. A B-level signed call from a known business usually outperforms an unsigned call from the same business — and an A-level call usually outperforms a B-level call. For a small business that depends on answered outbound calls, the level your provider can offer is one of the largest factors you can actually influence.

Sources

References used for this guide

• FCC call authentication

  US regulator overview of caller ID authentication and STIR/SHAKEN policy.

• CRTC caller ID spoofing

  Canadian regulator explanation of spoofing, traceback, and STIR/SHAKEN caller ID authentication.

• Twilio trusted calling

  Carrier infrastructure reference for STIR/SHAKEN trust authentication.