What impedance is the headset jack on the DuraFon?

The impedance of the headset jack is 32 ohms.  If your goal is to connect this to an external speaker or paging amplifier, you will need a 32 ohm impedance speaker or input for best performance.  Connecting to an 8 ohm load will result in low audio.  If the goal is to create a "wireless external ringer" (so to be heard in a very loud environment), then we suggest using a Viking M2W 2 watt external ringer and speaker.

"The impedance is a measure of how much earphones or headphones resist an electric current (more specifically, a change in a current)."