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)."