yea its silly simple, and has been a common hobby project for many years
example:
http://www.freecircuits.net/circuit-20.html
you wont really find much here as its just a handful of components. A single line phone uses two wires, typically red and green (red ring, green ground) when nothing is going on red is pretty much low, when you get a call in (ring) that joker will jump to 75-90 volts and has made me cuss many of times, and when your talking on it, its around 30-50 volts (all depending on what part of the world your in)