Perhaps seakazoo is referring to the 5 phone number limit per account? If there was no limit, then the amount of fake accounts would be kept to a minimum as they would just use the same account.
Maybe, but it would cause even more numbers to get wasted because users just love to click repeatedly on "Assign me a new number" for no reason. Either way, I think it's the wrong approach. Maybe some kind of call-back system to verify each new account ... and no to-be-called-back-number can exist more than once in the database ... but the determined fraudster takes even this little obstacle. Oh well...