It seems to be related to other stuff the US is behind on, but I've given up on finding out how it could be fixed in the general (last 20 years) political climate.
We had an effective program for a few years eliminating telemarketing calls on landlines.
Just keep a registry of every organization that you've given permission to to fake phone numbers, and which phone numbers they're allowed to fake. Make them route those calls through a special system and give them a secret token that that system will verify. Centralize your client/token/virtual phone number registry across all carriers (it's no different from certificate-authorities). If one of the accounts starts sending spam calls, revoke their token. Done.
STIR/SHAKEN is handling this at the provider level. There are an awful lot of PBX installations out there, with hundreds of makes and models and service lives in decades. You are absolutely not getting every business with a trunk line (e.g. essentially every business with more than one telephone) to participate in a protocol change.