This project displays telephone numbers decoded from tones. A microphone picks up the tones, a preamplifier boosts the signals, an SSI-202 DTMF chip decodes the tones, a Basic Stamp acts as an interface to an LCD display and also provides `RS-232` serial output. If you`d like to build your own tone decoder `from scratch`, read on: You might have trouble finding the SSI-202 chip, even though Radio Shack used to sell them. In the United States, the best place to buy them in small quantities is B.G. Micro, www.bgmicro.com. They have the SSI-202 chip for $2.25. If you have access to Mitel 8870 DTMF chips, you could use it in this circuit if you change the pins to match the corresponding functions. I have not done it, so I cant help you.