Using custom electronics and some simple microcontroller programming, you can hijack the normal function of a touch-tone phone, giving it a life of its own. The one described here was the central element of an office that was placed the gallery space. Visitors to the office would cause the phone to make two calls- one to the office itself, and one to a randomly selected number elsewhere in the country. The visitor to the gallery would answer the phone believing someone had called the office, often finding herself listening to one of the countless variations of "This number is no longer in service..." In other cases, he would hear a fax attempting to convert images into sound, or an outbound voicemail greeting for a person he did not know. If the visitor were lucky, or persistent, there would be another person on the other end- thinking the same thing as the visitor: "Someone has called me."
This will not be the most comprehensive look at physical computing, electronics, or any of the techniques used. I hope to give a clear enough picture that you would have an easier time pulling this off than I did, and more importantly, give a sense of other possibilities for communicating with the devices we use to communicate with one another.
I recommend Dan O'Sullivan and Tom Igoe's book Phisical Computing as a basic reference. There are also plenty of other great 'ibles here that will get you going on art and electronics.
Step 1: The Things
-Hot glue gun
and this non-exhaustive list of:
-Basic components-- resistors, LEDs (cheapo kinds are fine), 22 guage wire, solder, relays, FETs, circuit boards, electrical tape, other things I have forgotten
-Microcontroller (I used a Basic Stamp 2, though if I were doing it again, I'd use an Arduino)
-Old phone(s-- you'll be glad if you have spares to experiment)
-Variac (or other way to create ~90VAC from 110VAC)
-Electronic Ringer (new or scavenged) -- making a mechanical ringer ring separately is a project unto itself