Much like existing visual captcha's that want to verify you're human, you could have an intermediary service (think Google Voice) that would allow for a call to held, verified with an audio based challenge captcha (two+three equals 5) then pass the call on, if answered correctly. This could be a mix of questions or math problems to stymie voice recognition software.
It would thwart auto-dialers and put legitimate callers through as human - because after all, if we're going to be interrupted at dinner, I want to talk to a real person! (political, etc)
A lot of this could be integrated with Google Voice and the Obihai VoIP solutions already being used and sold. As Google Voice itself acts as a proxy with a phone # and allows for blocking.
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