I checked the FAQ, and couldn't find any specifics about how this line works. Then I searched the forum and found some interesting stuff related to hooking up a Nokia phone, posted by jheathco.

I just got one of those Handspring Treo cell phone/pda units, and I am going to get another headset and take it apart to create an AUX adapter. I would like to have the headset trigger the cell mute line, but I have no idea what signal to apply to this line in order to turn on the mute function. After the phone triggers the mute, I am going to use Hijack to switch over to the AUX input just like everyone else seems to be doing. My guess, based on a few comments in the forums, is that the cell mute line is triggered when 12V is applied to it. If that's the case I can either use some signal from the headset to trigger it, or I can simply wire a switch that applies 12V to the cell mute line from the power line.

Does this sound like the right approach, or am I missing something important? Do any of you guys using the empeg as a speakerphone see problems with feedback?