I know this is probably obvious, but have you checked to see if the modem's PNP feature can be turned off and manually set the com/IRQ? Then use either the modem setup feature of the hardware manager or look in the book and check which file to manually link to dev/modem. Also, if you do set it manually, in the BIOS of the MACHINE turn off the PNP for the IRQ you end up using (set it to ISA/LEGACY if available).
To be on the safe side I bought a standard ISA Modem, that has its own controller, to be used in LINUX. ALSO, maybe you can find better info at the site below. GOOD-LUCK!
http://www.o2.net/~gromitkc/winmodem.html