Polarity Guard. Corrects polarity problems caused by tip and ring reversal. Early touch tone phones lacked polarity protection and if they are connected to a phone line where the lines have been reversed, the phone will ring, you will be able to listen and talk, but you will not be able to use the dial. Polarity guards were introduced to address this problem, and from the early 70s forward, all phones contained built-in polarity protection.
Installing the Polarity Guard
To install the polarity guard on an early touch-tone phone you will need to remove the white wire that goes to the "F" terminal on the network and attach it to the terminal on the Polarity guard that corresponds to the green wire. The Green wire from the Polarity guard will then go to the "F" terminal on the network (along with the green wire from the tone pad.) Next, the white wire from the Polarity Guard must be attached to the "C" terminal on the network (along with the orange/black wire from the tone pad), and the other wire on the "C" terminal (either blue or brown) should be moved to the remaining terminal on the polarity guard. Be sure to carefully insulate the polarity guard (wrapping it with electrical tape works), or secure it inside the phone with double sided tape in such a way that it does not short against other components.