Google voice lets you do something along those lines. I personally found it incredibly tedious and quickly turned it off. I think the grandparents idea of *RC

Google voice lets you do something along those lines. I personally found it incredibly tedious and quickly turned it off. I think the grandparents idea of *RC is spot on. Verification can be done through volume; i.e. a one-off may do nothing but repeated reports indicate something is up. Just like reporting spam in email. Along those lines, some sort of charge-to-call system may work. Like calling collect in reverse, but the receiving party can decide to not charge the fee if its someone they know (or flip it, hitting # within 30 seconds of an inbound call will capture the fee and disconnect)