The factory NAV head has a mobile phone mute/audio input on it. I use mine for my GMRS/FRS radio when we do Dragon meets - the GMRS radio, when someone talks or when I transmit, mutes the AM/FM/CD and plays the FRS radio audio thru the passenger side speakers.

Works really good!

(This is the OLD STYLE of mobile phone mute/audio - back when you had an actual phone installed in the car. I know of no way of doing this with a handheld phone, not saying you could not do it, but you need audio out AND a mute line from the cell phone. Where ya gonna get a mute line? UNLESS the little box you have there has an audio-sensed mute function in it.)