I never added a phone number and thus the account got flagged immediately. Just shoot the twitter support a mail and they can unflag you.
Sorry for sort of OT, but my post on Reddit has been removed, and I hope that Twitter people might read this here.
Sorry for the noise everyone, I found out how to do it:
Do not register an account only with an e-mail address of yours and then ruefully append your phone number later because you are denied any usefulness of the account otherwise.
Instead, choose to register by your phone number in the first place right away, and pretend to volunteer to be easily trackable throughout the entire internet.