I am trying to make it work under win8...
It isn't a Prolific 2303 and it isn't an FTDI FT232 or similar...
Win8 does notice it was plugged in (it beeps) but nothing new (that I can see) appears under Control Panel / Devices.
I would expect a COM port with a Hayes compatible modem behind it. That is what their older models do. The old 7100 is a straight RS232 Hayes modem, but the XT is USB only.
The modem has a config for two things: Modem and USB. Neither makes any difference, but the 1st one suggests that perhaps it does have an RS232 interface...