Thanks for your answer, RequestedDeletion...
I set up the ttyI0 with the setup-program for dial-out (AVM Fritz Card), how I said, dial out works fine. MSN is also set, so I installed the dialin.rpm, so the mgetty.rpm. defined users with their corresponding numbers, added the ttyI0 to the Dial-In device and set the status to 'on'.
If I try now to connect to the number, I even don't get an answer. I tried to set the numbers in different formats (with prefix, with prefix without '0', without prefix), but nothing happens. I know that the dialing PC works fine, Tried it on another linux-machine with security enabled.
So far, so bad...
Thanks,
Chris