Is your modem an internal pci card modem? Hopefully it's not a winmodem.

Anyway I think it's probably best to delete whatever you have set up in kppp.
Then open a terminal window su to root and type "pppconfig"
Follow the program's set up guidence and save the changes. Then you can re-set up your kppp.

If you still can't connect after that enable the debug in kppp and copy and paste the errors.

Good luck