I put a fake phone number into Twitter and a throwaway email. Got a few complaints about not having verified my number, then got banned for breaking the rules.

“Your account is permanently suspended After careful review, we determined your account broke the Twitter Rules. Your account is permanently in read-only mode, which means you can’t Tweet, Retweet, or Like content. You won’t be able to create new accounts. If you think we got this wrong, you can submit an appeal.”

I can’t even follow anyone, but at least I can read tweets.

As I tweeted nothing nor indeed followed anyone all I can assume is that you have to have a real phone number to sign up.