It's a whole bunch of tradeoffs; requiring a working, non-voip phone number does raise the cost for abusers, but it's not enough to make spam unprofitable. Req

It's a whole bunch of tradeoffs; requiring a working, non-voip phone number does raise the cost for abusers, but it's not enough to make spam unprofitable. Requiring a deposit would be more direct, but administration of deposits would be a lot of work, and you have an uphill battle to convince users to pay anything, and even if they want to pay, accepting money is hard. And then after all that, some abusers will use your service to check their stolen credit cards.