If your phone will support data (most do these days, if not and you are on a contract it's time to upgrade) you won't need a modem, just the cable to the serial port of the pooter (if your phone and computer have infra-red you don't ned that either)
Just hook the phone to the serial port and tell WinDoze that you have a "standard modem" and you are up and running.
If you use an ISP with free or "local" call rates, be careful what the call rates are with the mobile!
------------------
What goes around . . .
. . often lands better!