I wanted to teach my Arduino to talk telephone, so I added a HT9000A DTMF chip to produce the familiar `touch tones` (upcoming projects will act upon `heard` DTMF tones, but with a different chip). The output is really low volume, so you may want to plug in amplified speakers or use an input on your stereo. You could build an amplifier
circuit with a LM386 as well. I used an Ham Radio program that listens for touch tones and displays the number it hears to verify operation.