Comment by smallerfish

> It is a way to increase usability for casual users, decrease spam by requiring some other source of identity tied to real existence (emails are easier to generate than throwaway phone numbers).

You either end up discriminating against users who have to use VOIP for whatever reasons (and there are legitimate reasons) by blocking VOIP numbers, or your barrier to entry for spammers is almost negligible. It's not a good system.

If you want to prove that users are humans, use a webcam and an id, or delegate the task to some bigcorp who already has a similar system. If that's too much for you in terms of privacy, you shouldn't be attempting to prove that users are humans in the first place. Maybe you should prevent spam via product driven solutions, e.g. whitelisted contacts.