I installed just a Motorola timeport handsfree and this is sending 12V out on the mute wire (so it would work with the empeg), and nothing when not in a call.

That mostlikely is the standard I think. Otherwise, if there would be nothing connected to the mute wire on the car stereo, it would be always in mute which doesnt make sense.

Anyways, I have just installed another Philips Carin navigation system, and there you can select if you would like to use 12V or 0 V for mute. J.