Hi Ian,
As our systems were designed and on the market before the Zumo's the AUX2 sockets are mainly there for music use so there is a 50% mute when the mics are opened.
So when a Part 1299 is being used on a phone call for example every time you speak with a person on the phone their volume is muted 50%, this make the calls hard to be heard.
With the BT dongle plugging into the AUX1 socket which is designed for phone use only there will be no muting at all on any calls, we then use a Part 1314 which is a 2m long isolated music lead that will run from the Zumo cradle and then to the AUX2 sockets on the systems to connect the GPS and music side of the Zumo.
Or you could run the BT dongle by itself but they are not designed for stereo music so the music will not sound the greatest but will run fine for the phone and GPS instructions if you were to run a seperate music source.
Rich