This is the original item installed into ATT and WECo sets that allowed early touchtone pads to work regardless of line polarity. If you have an early touchtone set (such as a 2500, princess, 2554 set) and it will ring and receive dial tone, but when you press the buttons nothing happens, you will need to install one of these to allw it to work. It is a rectifier for line + and - polarity. They are easy to install: take the white and brown wires off the switch (from C and F) and install them on the screw contacts on the guard. Then take the GREEN lead off the polarity guard and put it on F and the WHITE lead off the polarity guard and put it on terminal C. Cover with electrical tape and tuck inside the phone. A simple repair! These are NEW OLD STOCK. Shipping $1.