You can work out whether your modem's at fault by connecting it to one of the Windows boxes you've got lying around, using the "default 56k" driver and hyperterm or phone dialler just to try dialling numbers, or any ol' internet account to see that it successfully negotiates a connection.
But from the sound of it some settings somewhere are wrong. There is (buried somewhere in the various menus) a program called either KDial or KInternet Dialer (I forget, and can't check anymore
)- it circumvents DrakeConnect (and YaST2 incidentally) and keeps its own config files etc for creating dialup settings. You might try that instead - it tends to work where the other alternatives fail.