I don't beleive that you can use a headset via the AUX jack on a Partner phone connected to an ETR card. You lose the combination extension operation on a Magix. So you're going to have to go to something that connects through the handset jack, gives you a place to plug the handset back into, and has an "A/B" switch to go to handset/headset. AND, you're going to need to lift the handset out of the cradle to activate the phone, as you can't put Headset Status/Hangup buttons on an ETR phone, like you can with a MLX or TDL.