This is a good quality high wattage 3.3K resistor for converting 300 series, 700 series and Trimphones to run at one Ren and so consuming the same current as a modern telephone. Without this fitted your telephone will hug all of the available Ren value and so may prevent other telephones on the line from ringing properly. I have fitted crimp fork terminals and soldered them to make sure of a good electrical contact. Make sure if fitting to a 700 series you have 1000 ohm bell coils. They will be marked 500 on each coil. If they are marked 2000 on each coil this unit is not needed.