I had the same happening, created an account and it was locked the same day only to be unlocked when I provide a phone number. I wrote them an email telling them I refuse to give them my number for privacy reasons and they unlocked my account the next day.

Which email address did you use to contact them?

I've had this exact same problem about half a dozen times, and every time I have difficulty getting in contact with a real person, and not being led through an automated system that just ends up asking me for a phone number, or getting a reply hours later about how that inbox isn't monitored.

Sorry, it's been a while, I don't quite remember, but I think I either replied to the mail they sent informing me of the lock or it was a support contact address in that email. And they never replied, I just logged into my account the next day and the lock was gone. I wonder if a real person ever read my mail or if I simply solved some sort of CAPTCHA by writing that mail.

I'm pretty sure asking for the phone number is standard

But then why not ask for it when registering?

Well, of course to decrease friction and avoid turning people away at the entrance. You get smacked in the face with the request for your phone, once you've followed some interesting people, interacted with the site a bit and will form some attachment. Then you are more likely to enter your phone to keep using it.

There's a psychological effect of feeling more strongly about retaining a given amount of already owned value compared to gaining an equivalent amount of stuff you don't have yet.

You wouldn't pay with your phone number to get a Twitter account, but you would pay with it to keep an account you already have and started using.

Dark patterns.

I'll tell them mine if they tell me theirs.