If you have a Treo 300 or Treo 600, check out
wmodem. I currently use it with a Treo 600 on Sprint and my Powerbook. Works very well. It only works with Treo's, however. Don't know what kind of phone you have.
Also, go to
Treo Central if you want a forum that would better pertain to these types of things, if you're a Treo owner.