Dial up connection doesn't work (Hagenuk DataBox, ISDN Terminal Adapter on serial port)

Hello, Since over 7 years I'm testing Linux and Suse, and I would have been surprised, if the setup of the dial up connection would work without any trouble! I configure the dial up connection with Yast2, and my modem is recognized correctly (AT ISDN terminal adapter on serial port ttsy0, init string ATZ and ATB8 for PPP, connection speed fixed to 115200). Then I choose the MSN internet connection. But when I start Kinternet dial up, the modem is not initialized, no connection will be established. I try this now since hours, and I don't know, what else to do. With Windows, I never had this trouble, with Linux I have it each time I install it. Maybe this is the reason, people are frustrated about Linux. Moritz