Complicated circuit for line working telephone

 I found this old line working telephone in the museum of the South Tynedale Railway in Alston.

I've been looking at the rather complex circuit diagram and trying to make sense of it. At first I assumed that the blocks of 4 diodes would be bridge rectifiers. Underneath it calls this section a "regulator". It turns out that if you put a diode into a series circuit with a resistor such that they share the p.d., even if the p.d. changes the diode will always take just 0.7V because that is its forward bias to conduct. This fixes the p.d. across the diode and regulates the voltage. I think that RLP probably stands for "resistor in low pass circuit". If you follow the wiring across to IC you find inductances - loops of wire. When AC flows through such a wire, the changing flux that is set up links its own coil so causes a back emf to be induced. This acts against the current. The higher the frequency the worse it gets, so you can use it to allow low frequencies through.