With kde, you can use kppp. In the setup, try various serial ports to see if they work:
/dev/ttyS0 (which is COM1)
and so on.
Click the 'Query Modem' button to test the port. If there is a modem in the port, it will reply. If there is no modem there, or the modem is a Winmodem, you will get something like: 'Modem is busy'. See www.linmodems.org
for help with winmodems.