Don't waste your time. My dealer also told me it should work. After persisting for 2 months I got with Garmin tech support (also a rider with a GPS) who knew the answer right away;
You need a Nolan or some other bluetooth type helmet or do the work-around as others have posted.
Motorola HF820 BT speaker with a 2.5 mm male x male adapter pluged into it and the Voice II cell phone input.
Pair the Nav III with the phone. Pair the HF820 w/the Nav III but NOT with the phone (or at least keep it disabled from the phone). Set the volume on the HF820 up. It will work, I can hear great BUT my voice out to the caller is garbled. BTW, even if I can't talk out I like knowing who is calling in (the Nav III displays who is calling) and I can decide if it is required for me to pull over and make a call.
Family code= three call in a row by same caller = emergency
I now plug my phone innto the voice ll cell phone input, disable the bluetooth, and set to auto answer if I need my phone while riding.