In Australia, this would not be a good idea. I know several people who got caught using illegal home-made plugins for their phone line. They all paid heavy fines. Also, in Australia, the voltage jumps from its normal 50V DC to a huge 200V AC when the phone rings. See why it isn't such a great idea after all? Unless you can be bothered fitting your box with automatic switching and protection circuitry.

ACTUAL PROOF OF WHY THIS WON'T WORK:

The calculator also needs a certain amount of current to run. The amount of current coming from a phone line is less than 1mA--not nearly enough to run the calulator. Also, I don't see how the phone line will be busy when this thing is hooked up. One of the 4 prongs need to have power applied to it. US residents: A 4 prong modular jack will do you nothing just go to radio shack and get a phone line extension. This will save you time and money(it won't really save you money, because in the end your calculator will be destroyed.) Another thing for all you out there who think this might work. The incoming voltage(from the phone company) is alternating current(AC), the batteries in the calcuator are direct current(DC). You will need a full-wave bridge rectifier to make this work(this is 4 silicon diodes.) This will definitely jack up the price of this phone line supply to about 15USD, depending on where you live. You can also get rechargable batteries AND the recharger for about 16.99USD. So, in conculsion, THERE IS NO WAY THAT THIS THING WILL EVER WORK USING THE PLANS THAT LUIGI SUPPLIED, AND IT WON'T WORK NO MATTER HOW MUCH YOU MODIFY IT. IT WILL ALWAYS BE CHEAPER AND COST EFFECTIVE TO BUY RECHARGABLE BATTERIES.