I think you may be overthinking it. In Sweden we already have a registry where we can opt out of telemarketing[1] (as well as ads in our physical mailboxes[2]). Generally this works well. Yes I still get a couple of "this is Microsoft calling" in Indian accent per year. Two spam calls per year isn't bad though. And this has worked for decades here, without special technical measures.

(Companies with which you already have a prior relation are allowed to call / mail you though, but this is always possible to opt out of.)

[1] https://www.nixtelefon.org/

[2] https://nixa.nixadresserat.se/