Hi Paul, 1st of all you did not mention what verison of Windows 98 workstation you are running! It is best to upgrade to Windows 98SE for free at Windows Update and get the latest updates and driver for Windows 98SE.
Another thing you need to getthe latest drivers for your Faxmodem from the manufacture of the modem. Most of the time Windows 98, it does not matter what verison uses a generic dirver, and does not support all of the features of your FaxModem, unless you manually install the latest driver.
One other suggestion is to get a newer modem with at least a PCI Bus, the prices are very resonable, and probalble cheaper than what you paid for the ISA Modem years ago, and are designed to work with Windows 98.
Good luck and havea great day.