Hi, op here. They're asking me for any phone number, not for one tied to the account (there is none). I've confirmed this by comparing with the message a friend sees with two factor authentication turned on.
Some people are posting here saying they got in using a stranger's number so I still don't understand how providing a number proves who I am.
Some people are posting here saying they got in using a stranger's number so I still don't understand how providing a number proves who I am.