Found this driver on the 3Com site, after pulling my hair out looking for Model 0484. This is the driver for that modem, which 3Com refers to as Model #661749-81. Not only did it work for me, but it actually sped up my dial up access.
You'll need to unzip this to a floppy, then replace the driver, if you currently have one. You may get a message about "The best driver is installed.", but I ignored it, and got the desired results. Send me any questions. Good luck!