What aspect of it doesn't work? Does the machine not pick up the line? Does it not recognize the fax tone? Or do you get errors sending or receiving faxes?

VoIPO says that their lines are "tuned" for voice and not for modems or faxes. Some people manage to get fax machines to work - you could try setting the speed lower. I have also heard of the trick of inserting a DSL filter between the fax machine and the phone line. There are no guarantees.