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Twitter lets you register with just an email address, but will require a phone number to continue to use the account almost immediately upon actually using it.

US phone numbers are easily available for under $1/month from various services, and you don't need the number for a whole month. They're cheaper if you're willing to do some more legwork.

Requiring a phone number reduces casual abuse, and increases costs for abusers, but is only really effective if the abuser's gains are in the same neighborhood as the costs, in which case they'll move to somewhere they can gain more or spend less.