MTK - had exactly the same problem with a 'see-through' (very modern!) 56K USB modem. Turned out it operated like a WinModem in that it was software controlled. Tried everything - very poor performance.
Sent it back and exchanged it for a 56K ISA modem, and that sings along with generic WinME drivers. Best speeds I've ever had.
Hope that helps a bit (just a biased opinion!)
