Usually you would be using a car docking adapter specific to that model of cell phone. The car docking adapter would contain the necessary mute wire.

If you don't have the mute wire already sitting there waiting for you to use it, then you're going to have to come up with a clever hack of some kind. The empeg doesn't care, it just responds to the presence of the voltage (or lack of voltage) on the mute line (configurable in emplode).