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So the problems with the phone numbers is how easy it is to swap numbers. Say you make an account with your real number then hack and get banned. Bit whoop go onto google voice, or any other phone service even the paid for ones, and grab a unclaimed number link it to your normal number and there you go. Get banned again? Just grab another number. Number already banned from other hackers using it? Just get another. By the time they run out of numbers to use new players probably wouldn't be able to play.
So defiantly need a more static form of verification.