LeorickSimon writes in –
Using your old phones, you could create personal “micro” telephone exchange as simple as above circuit. I manage to solder the test circuit less than a minute. Than connect the phone and test it. It works. So why 39 ohm? From Epanorama Dot Net, the correct value of the resistor should be a resistor that can limit the circuit current around 30mA total loop. The method is by replacing the resistor in the circuit with a variable resistor, plug in the phones, apply power and tune the current with the variable resistor so the current flow is 30mA. After that take out the variable resistor and measure the resistance/ohm across it. Find the fixed resistor with the nearest ohm reading to it and replace it to the circuit.
My Electronics Hobby: Simplest Intercom – Link.