Okay, you already know about using the serial port on the back of the TiVo and changing the dialing prefix and all of that.
If you have a PC with a 9 pin serial port (or even a 25 pin and an adapter) with an internal modem (and if not you can probably find an internal modem dirt cheap on Craigslist), there's a freeware program called pilotnet that was designed to let PalmPilot owner's use their computer's modems.
Of course when you Google for it you'll have to wade through a lot of airplane pilot related results.