You **could** use the Ascii value of each character (8 bit byte), and a simple routine to equate to a unique frequency. That's a total of 256 individually distinguishable tones. There are spectrum analyzer/oscilloscope/tone generator examples on PlanetSourceCode.com which you could use as a starting point.

However, as I stated before, it would be far better to use existing modem technology. There are good reasons why data isn't sent down the phone lines in the way you describe. It's not just a matter of frequencies for bytes. Consider the S/N ratio of HAM just for starters. Even if you get it to work, it will be quite slow. Though a good learning experience, and perhaps a lot of fun, the method you seek is impractical. From your descriptions it seems like you are looking for something others would want to use, even purchase. Please understand that I'm not trying to put down the concept. In fact, I've tinkered with a number of things along these lines. Transmitters, receivers, companders, etc, using ultasonics, Infrared, and basically all over the radio spectrum. It's great fun, but in a practical application, you'd need better (if you even get it to work that way). I'm simply pointing out a far more viable solution, trying to save you trouble.

A modem already does the toughest parts for you. I'm fairly certain I've used some modem protocol to send data without waiting for a reply. This could work in your application, and the modem wouldn't even know there wasn't a phone line.

Whatever you do with this, good luck and of course have fun!