[Dave Moll]

If the third wire is "anti-tinkle", it may need to be connected to the pin that goes to the ringing capacitor in the master socket for the 'phone to ring - but best to start as suggested above by Graham, then only go down this route if you find everything else works except the ringer.

Again the easiest way to check which of the two inner wires is connected to the capacitor is by trial and error. Beware, however, that an incoming ringing current packs a punch if it passes through the human body - not lethal, but painful!

edit: Tony beat me to it while I was composing my post.