I'm always hesitant to add numbers to my phone's block list because they spoof numbers often and I'm worried that I'll block a number that legitimately needs to reach me in the future. I know the chances are tiny but it would be a huge PITA to debug that issue and if they do reach me through some other means I doubt I'd remember to check my block list to see if I happened to block them in the past.

I just added the fourth point about blocking unknown numbers. If your buddy is in a situation (e.g. certain type of job) that makes this impractical, then I'm afraid there's no better solution yet.

Edit: Actually, Google Pixel phones have an automatic call screening feature using the Google Assistant: https://support.google.com/phoneapp/answer/9118387. Demo: https://www.youtube.com/watch?v=jWap-nPQUO0. The privacy trade-off is that Google screens all of your calls, and the feature does not work with custom OSes, but some people might find this to be worth the added convenience. I'd love to see a FOSS implementation of this after a free voice recognition solution becomes available.

I thought the whole SHAKEN/STIR[1] push would help. I think what happened is that it pushed out the amateurs. So the pros just amped up their volume.

It's still pretty new though. So maybe it will start working better soon.

[1] https://en.m.wikipedia.org/wiki/STIR/SHAKEN