I own one too, and have the same exact problem. Infact, since my phone cord that connects to it upstairs is "too long" it won't even connect (my supra fax 22.8 modem works perfectly upstairs, so I stayed with it). I tried it connected to the downstairs line and it worked, but not at 56k speeds. Since my computer is upstairs I've left the crappy modem in it's box to collect dust. Truly a waste of money!
When I first got it I thought that the unit was defective and sent it back. Then, when the new one came and had the same problem, I knew that they were all made poorly.