The number that the call is coming from

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A Typical STIR/SHAKEN Workflow
When someone initiates a phone call, the calling party issues a SIP INVITE, which goes to the originating service provider. Once received, the provider checks the source of the call and the calling number to determine their attestation level. There are typically three options available:

In Full or “A” Attestation, the service provider knows who the customer is, and can vouch for their right to use a particular phone number.
In Partial or “B” Attestation, the service provider knows the customer, but does not know the source of the phone number.
In Gateway or “C” kuwait telegram Attestation, the service provider cannot authenticate the source of a call (which might for example, be an international gateway) — even though the service provider originates the call onto the network.
The originating service provider will use an authentication service to create an encrypted SIP identity header. This consists of several elements, including:

The receiving number
The current date and a time stamp of the call
The attestation level
A unique origination identifier, which aids in tracing back the call
After this, the SIP Invite and SIP identity header are sent to the terminating provider, who passes the SIP invite to a verification service. If the call is successfully verified, the terminating provider makes a final decision on whether to complete or block the call. In making this decision, they will take the attestation level into account, as well as other factors such as relevant information contained in their own call analytics.

How It Works in More Detail
Behind the scenes, a typical STIR/SHAKEN implementation consists of several components. They include:

The STI-Authentication Server (STI-AS): This provides an Application Programming Interface known as the REST API, which is responsible for signing requests. To this end, the API has access to private keys in the SKS (Secure Key Store).

The STI-Verification Server (STI-VS): This provides the REST API that plays a role in processing verification requests. This API also retrieves public keys from the public internet using the URL contained in the verification request.

The Authenticator: This is the component in the carrier network that invokes the Authentication and Signing Services to create and verify digital signatures.