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.

For anyone else with this issue, try: https://help.twitter.com/forms/signin