There are exceptionally few times that Windows will allow you to modify the IRQ of anything. A modem does not fall under that exception. I know that all Windows tells you to do is uncheck a box and everything will work but that is a lie - a big fat whopper of a lie born of Win95.
The best thing ever to happen to IRQs was the change to legacy free motherboards. Now they are not limited to 15 IRQs (of which really only a handful are actually available to use).
EDIT// As far as I am aware Windows virtualises the ports anyway so the BIOS and Windows allocation tables do not match.
AMD Mobile 2500+
A7N8X Deluxe rev2.0
1GB Corsair XMS PC2700
GeCube Radeon 9800 Pro
Seagate SATA 80GB
2xSegate 160GB in RAID0