This is a neat little project which involves making a Red Box from a standard Radio Shack tone generator. What's a Red Box you ask?? Well a Red Box is a device that outputs the nickel, dime, and quarter tones used in a payphone. Theoretically one could use such a device to place phone calls at a payphone for free. These were very popular in the 90's and may not work on today's new phone system. Although I have heard they will still work in some remote areas where time has forgotten ;)
Check it out Here.