I have a USB NIC connecting a DSL modem, essentially the same hardware setup, and it works fine. Possible issues include your MB's support for USB, and whether updated driver and/or BIOS needs to be installed; MB and OS configuration of USB bus; whether a PCI adapter is in the PCI slot that generally shares PIRQ with USB bus (slot 4, I vaguely recall); your OS's support for the modem's driver (may need an update). The last two are the most probable, since the first two would likely cause it to not work at all.