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Eh, the idea obviously isn't to make it impossible to troll. Just to make it more trouble than its worth for some meaningful fraction of potential trolls.

> I guess Twitter hasn't ever heard of Google Voice. Or prepaid phone numbers.

I've run into services that won't take either. They're in known blocks of numbers.

Craigslist is pretty effective at stopping those types of evasion. They ban VOIP phone numbers, for instance.