[WallyFunk]

Twitter has been compromised by a Sybil attack[0] many times. They tried to stop people registering sockpuppet accounts en masse by forcing phone verification, but phone numbers are trivial to acquire with things like Twilio and other services, so it's a minor hiccup for the troll farm operators.

[0] https://en.wikipedia.org/wiki/Sybil_attack

[skinnymuch]

I thought it was relatively easy to block off VoIP numbers.