I had this problem when creating a new Twitter account last month. Within minutes after setting up an account via iOS app, was locked out for "suspicious behavior". Didn't even tweet once. Residential IP. They wanted me to add a phone number.
I went to the help section and wrote a pissed off message to customer support...if they wanted my phone # they should have asked for it instead of accusing me. Shortly after, my account started working again.
I had the same thing happen. During signup it asked for my phone number but said it was optional. A couple of minutes after I created my account, it was locked and they said it required a phone number to prove I wasn't a bot. I just closed out my account because I only planned to use it to follow some people anyway.
I wonder how anyone actually uses twitter because I constantly get messages that I am rate limited/blocked and when I signed up for an account it was quickly locked when I had posted nothing. Its like they actively want no one to use it.
I went to the help section and wrote a pissed off message to customer support...if they wanted my phone # they should have asked for it instead of accusing me. Shortly after, my account started working again.
For anyone else with this issue, try: https://help.twitter.com/forms/signin