Definition
STIR/SHAKEN is the framework US carriers use to sign calls with a digital certificate and verify that the caller ID is genuine. A signed call carries an attestation level that says how well the carrier knows the caller.
How STIR/SHAKEN works
STIR is the IETF set of standards, SHAKEN the telecom industry profile for using them. The originating carrier creates a small signed token, a PASSporT defined in RFC 8225, holding the calling and called numbers, a timestamp and an attestation level. It attaches the token to the call’s SIP Identity header, as described in RFC 8224. The terminating carrier checks the signature against a certificate. If it matches, the call is marked as verified.
There are three attestation levels. A means the carrier knows the customer and that the customer is authorized to use the number. B means it knows the customer but cannot confirm the number. C means the call only entered its network at a gateway, for example from abroad, and the carrier knows nothing else about its origin.
The FCC required carriers to implement the framework in the IP parts of their networks. The signature shows whether a caller is authorized to use a number. It does not say who is calling.
Why it matters for voice agents
An outbound voice agent does not sign calls. Its carrier does, and the level depends on what the carrier knows about you and the numbers. Calls that arrive with A attestation are more likely to be shown as verified, while calls with a lower level or none are more likely to be labeled or blocked by the receiving carrier or phone. Call labeling by apps and carriers is separate from STIR/SHAKEN and has its own rules.
Practical steps are boring. Give your carrier accurate business details, use numbers assigned to you and avoid presenting numbers you do not control. If you use a SIP trunk, ask the carrier which attestation level your calls get.
Signing proves a number is authorized for the call. It does not make the call lawful. Outbound calling rules apply either way.
Example
An illustrative check: you place a test call from a number on your trunk to a phone whose carrier displays verification status. The call shows as verified. You then place one from a number you do not hold at the carrier, and the call may arrive unverified or labeled as a possible spam call.