Internal PCI modems are always a pain, certainly going back that far. Try checking out
http://www.linmodems.org for chipsets that have Linux drivers available, which also gives an overview as to the technologies in PCI modems and why they're so problematical. Typing "lspci" will show you extra info on you PCI modem including the chipset it's running, which might help.
Failing that, the usual recommendation is to purchase an external serial modem which aren't that expensive, and will work 99% of the time straight out of the box with no fiddly drivers.