Maybe that was possible 10 years go, they now require that you put up a web page for your domain with a valid German imprint (most importantly: your full contact information).
Note that unless T-Online has additional requirements here this doesn't need to be your home address but only a valid mail address through which you can be reached.
To be fair, I had that in place 10 years ago (and if you already have a mailserver, it's trivial (as in: MUCH easier than to set up a mail server) to host a small imprinted HTML page).
