See here: :
Model MSP5900-U(X), Best Data 56K Mach 2 Model A56FW-PCI, PCTel PCT789T-A chipset, requires PCTel "Linmodem" driver (PCI\VEN_134D&DEV_7890&SUBSYS_0001122D)
Linmodem
Best of luck. There are 5 different models of this modem, the only way to know which chipset you have is to open it up and read it off the IC, if you have the Conextant chipset, it's Windows only.