STIR/SHAKEN is a Carrier-controlled function. You can't make changes yourself as the carrier is vouching for the information. Basically, attestation A is attached based on if you are a customer of the carrier and using one of their DID's. Attestation B is the carrier knows you, but does not recognize the DID. C is good luck getting your call to connect!
Genesys has a process for changing the attestation and what is provided to the other carriers. Inbound is something I would like to see so if we get a call with Attestation C, we can dump to a recording.