Glossary · Compliance

What is STIR/SHAKEN?

Also called: attestation a, call signing

Reviewed by Sujan ThapaliyaLast updated August 18, 2026

Definition

STIR/SHAKEN is the framework US and Canadian carriers use to cryptographically sign calls so the receiving network can tell whether the caller ID is trustworthy. Calls are signed at one of three attestation levels, and the level heavily influences whether a call is labelled as spam.

In practice

How stir/shaken actually works

Attestation A means the originating provider knows the customer and confirms they have the right to use that number. Attestation B means the customer is known but the number's provenance is not confirmed. Attestation C means the call was simply passed along, with no verification.

Only calls from numbers you actually own can earn Attestation A. This is the concrete, technical reason that renting numbers from a shared pool produces worse deliverability than owning them.

STIR/SHAKEN does not stop a call being labelled on its own; it gives analytics engines the signal they need to trust your caller ID rather than treating it as suspect.

A shared number pool caps at B by definition — the provider cannot assert what is not true.
Attestation	The carrier asserts	When you get it
A (full)	It knows the customer and they may use this number	Numbers provisioned directly to you
B (partial)	It knows the customer, but not that they own the number	Shared or pooled numbers
C (gateway)	Neither	Traffic handed off from elsewhere

Worked example

A call from a number provisioned directly to you signs at A. The same call placed from a pooled number your provider also lends to other customers signs at B, because the provider cannot assert you are entitled to it.

STIR/SHAKEN: common questions

What do attestation levels A, B and C mean?

A means the provider knows the customer and confirms they are entitled to use the calling number. B means it knows the customer but cannot confirm the number. C means it can confirm neither, typically on traffic handed off from elsewhere. Terminating carriers weight them differently when deciding what to display.

Does Attestation A stop calls being labelled as spam?

No. Attestation says the call is genuinely from the number it claims; it says nothing about whether the calling is wanted. Analytics engines still score volume, duration and complaints, so a fully attested number can still be labelled.

Why can a shared number pool only reach B attestation?

Because full attestation requires the provider to assert that this specific customer is entitled to this specific number. On a pool the same number is used by different customers over time, so that assertion cannot honestly be made.

Sources

Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See stir/shaken in the product

Branded caller ID puts your business name on the recipient's screen instead of a bare number. In North America it works through CNAM, the caller-name record attached to a number, combined with STIR/SHAKEN signing that lets the terminating carrier trust the call is really from you.

Explore branded caller id Get early access