PPP connection: what's wrong with it?



I'm looking for consistent advice about PPP connection through modem (I've gone through many forums, no answer so far...). I've got an ISDN Zyxel 2864I (500$, hell). The Zyxel modem script for OSX doesn't exist, and the Zyxel Comp. say they don't plan to make any. I can't connect through Classic (Eudora, Netscape) and no built-in modem script in X works either. So no way to connect my computer to the net. HOWEVER, in the OSX PB there was a script called "Hayes compatible" which allowed me connecting, even though at a very slow (56k) rate. Does anyone have a suggestion before I trash my modem? E.g.: editing a modem script, finding the "Hayes compatible script", other major PPP issues and tips?