Phone confirmation is a very sure way the person on the other end is real and unique.

It is harder to get more phone numbers than it is to get more email addresses.

These sites use phone confirmation.

When signing up to a new account, will call you and speak the four-digit code

Then, you get +2 reputatation

When signing up too many accounts (10+?), Google will require a text message (anytime) or voice (during business hours) five-digit code confirmation