they claimed my behavior was suspicious (I hadn't posted one thing, was setting up the profile). Demanded I give them my phone number.

This is actually a part of an ongoing effort to tie real world identity to social media accounts. FB has been deleting accounts for not having real human names and it has been going on for a while. To scale operations hey are just using algorithms. The more focused your account look like the more likely it is to get flagged by algorithms. I remember not being able to keep sharing FB posts from a single account for more that few times in a row.