The real answer to the problem is to deprecate the legacy telephone system. It will never be as secure or user-configurable as just about any modern implementation of voice/video over IP.
The legacy telephone system is being deprecated. All three US mobile operators now have VoLTE (ENUM) interconnection with each other. STIR/SHAKEN call verification is happening between the mobile operators and large consumer VoIP operators like Comcast and Charter. VoIP is far cheaper to operate than POTS and most all operators are using it now and shuttering their legacy networks.
The issue has to do with regulations around the phone system. Rural call completion to small operators is a requirement - as it should be - but has loopholes that encourage abuse. This rural call completion regulation also comes with the ability for small operators to charge certain prices for call completion so they can afford to keep their high-cost rural customers serviced. The larger carriers pay these fees to connect their subscribers to those of the rural carrier.
However, some smaller operators have also been using these higher rates as a means of profiteering by allowing massive amounts of spam traffic through their networks towards the larger carriers.
The real answer to the problem is to deprecate the legacy telephone system. It will never be as secure or user-configurable as just about any modern implementation of voice/video over IP.