Found this particular driver at www.aceex.com.tw on 20th Dec 2000. The other ones on the driverguide site up until now caused a BSOD on win2000.
We didn't know this was an isdn modem until we looked up the winbond chip number!
The driver is for a passive pci isdn modem and might sort your problems out.
BTW Win2000 finds it as a network controller but that is par for the course.