I don't have a switch on mine. I bought "something" from Jon, he told me what I needed, some sort of cellset. My Razr phone is Bluetoothed to my Zumo. When the phone rings, I hear it over the intercome, touch the Zumo screen to answer and it automatically mutes any other audio until the call is ended.
If you are using the 'Multi-Output' cellset kit, it includes a switch which allows you to send the GPS audio (voice, phone and music) to either the AUX input (for listening to music) or the intercom (if you want the GPS audio to be able to override any selected audio chosen). This is also the most-convenient method to use the phone interface (IMHO).
Since the switch doesn't have to be activated very often, mounting it inside the LH fairing pocket will keep it relatively accessible and out of the way.