It's supposed to be wired to a car hands-free kit (the ones that are installed all over the car - £200+ quid or so). It automatically mutes the stereo (or attenuates a lot) when there's a phone call in place.
I suppose you could put a switch on it, but all HU's I've seen that have the wire also have an obvious front panel switch that you press to get the same effect (labelled ATT or something like that).