If this is a PCI modem, try moving the modem to a different slot.. perhaps it might change the com port that it uses. Sorry but I used to know an exact answer to this. I did this same thing on an old Compaq Presario 7200 that used an ISA 14K baud modem... upgraded to a high speed 56K PCI modem and had some issues...
but now I cannot remember how I fixed it, but it was related to changing the COM ports.