DTMF, four capital letters, printed in bold, stands for “Dual-Tone Multi-Frequency Signaling.” These are the sounds of a touch-tone telephone. And they’re quite easy to create, as they are nothing more than two sine waves of different frequencies being added together. The frequency chart is listed at Wikipedia.
You can download the Csound source code here.
I like this example for a few reasons. First and foremost, it’s simple. This is the minimal possible additive synthesizer achievable, using only two sine waves per note. DTMF tones are also a fundamental part of our telecommunications infrastructure. And this ties in perfectly with the history of computer music, since the first digital synthesizer, MUSIC, was developed at Bell Labs in 1957 by Max Mathews. Be sure to read up on Max.