As long as there's no spoofing, shared client-side software-level blacklists (i.e. the same "adblocking" we do elsewhere) can handle the rest.

It would solve 99% of the problem just to be able to block all the calls that either 1. originate directly in other countries, or 2. that originate from number-ranges leased to carriers known to exist solely for the purpose of leasing numbers to VoIP/softphone/"app calling" providers; or 3. that originate from numbers that refer to the PBXes of companies whose business is to proxy foreign numbers to appear as local numbers. (Heck, once there's no more spoofing, we'll notice how big of a problem those are, and we'll probably get a law against them.)

How does client side filtering work for unknown or private numbers? I tend not to pick these up anyway.