Hi, I also use Mandrake 8.0 so I may be able to help if you give me some more info as below:
Which dialer are you using (Kppp)?
Which modem are you using i.e internal/external model etc.?
Have you double checked your modem set up and used the test function in the setup to check that the modem is working and getting a dialtone?
Let me know and I'll see what I can come up with to help you OK?