Here is a quick cool thing to look at. I really want to design something using telephone touch tone. It is 2 frequencied combined to make the tone, and here is an article on getting a encoder and decoder. I will do better updates after the holidays. Things are crazy now, and I am at my parents house and on dialup. It is like going back in time 
Anyway, check out this article. It is very good.
…Dual-tone-multi-frequency (DTMF, also known as touch-tone) are the audible sounds you hear when you press keys on your phone…Touch-tone is familiar to many (telephone), it is a mature technology, and readily available with off-the-shelf, single-chip, low-cost components. For these reasons DTMF is often used in remote control applications that typically use telephones (e.g. accessing your messages from an answering machine, retrieving your account balance info from your bank’s database)….
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[...] I have already talked about a DTMF decoder, and some of the many uses for one. I ran across a short article, that sparked my interest in these chips. What does DTMF mean? It means “dual tone multi frequency”. Who would have thought that a dial tone on a phone was actually two different frequencies that were modulated together? Why would they do this? Well the reason is that this gives a powerful way to send data, without the chance of interference. If there are only several different expressions that need to be sent, then this is a nice way to do it. There are only 8 tones that are modulated. There is a high tone, and a low tone in each set of modulations. This gives a total of 16 possible modulated tones, of course a telephone keypad only uses 12. Anyway, Raphnet.net found an old answering machine and pulled out the CD22204 chip. This will demodulate the codes. He connected it to a microcontroller and connected that to a serial port on a computer. He wanted to keep track of what phone numbers he was dialing. [Link] This is only one of many possible uses for this chip. It could be used for building an IR remote. You could build a transmitter that used an IR LED, and then transmit signals to an IR sensor. You simply decode what you are picking up, and connect the chip to whatever you want to control. Another project that I found was a DTMF decoder that was connected to a microphone. This microphone had a pre-amplifier, which connected the signal to a SSI-202 DTMF decoder chip. He then connected this to a Basic Stamp which sent out serial data to a LCD screen. He basically displayed what numbers were being called by what it heard. [Link] Anyway, I have already discussed some uses with this chip in a previous post. I linked to an extensive overview on how this chip works. If you are interested in reading more about this, you can find my post here. [...]
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