"Phillip Torrone (of Make & adafruit) talks about how to hack a traditional pay phone, connect it to a computer so it can be used with Skype, modify it so it requires quarters, or even make it so it requires a red box to simulate the sound of quarters dropping in". Not many technical details, though...: http://www.youtube.com/watch?v=sR4IGmCrJUg
From Wireless World: "In 1969 the first non-experimental cellular call was made from a service running on Metroliner trains from New York City to Washington DC, where passengers found that they could "conveniently make telephone calls while racing along at better than 100 miles an hour" (Paul, 1969). This also means that (ironically) the first cellular phone calls were made by a payphone" (p. 8).
Payphone project on github: configuring a (dumb) payphone to make and receive calls through an Asterisk PBX.
Instruction videos (by Conway668):