If you can, use serial PPP through your computer.
You can try the dial prefix:
,#136 (forces 33.6 connection on internal modem)
,#128 (forces 28.8 connection on internal modem)
,#119 (forces 19.2 connection on internal modem)
, is the pause buttom, # is the Enter button on the lower right of the number pad.
Another trick might be to use a dial prefix of #99,
That sets some VOIP ATAs it "Fax" mode, which allows modems to work better.