Some known problems you may experience with Garmin gear
1. Most Zumo 600 Series GPS have a problem where the external mic jack has a plug in it, but the built-in mic does not disconnect, and you end up having no voice through your wired connection to the mic input, only what noise might hit the unit itself and the mic on the front of the body of the GPS. Sometimes it depends on what you plug into that jack, and sometimes, the sensing funciton simply does not work, and that's unfortunately a call to Garmin to get it sorted out.
2. Connector pins on the Zumo series are gold plated, but that does not mean they can't get crud on them. They won't corrode, but dust or pepperoni can make them have intermittent connections. On the 550 cradles, you can open the cradle and find two flat connectors that can be cleaned in severe cases. They are not gold plated, but copper, and I've been in mine before and fixed an audio out problem by simply rubbing with a pencil eraser. Don't spray or use greases on them. My cradle is ancient, yours may have newer innards.
3. Some phones work better than others. If you're not happy with your phone calls through the GPS, try another phone. Most have excellent performance, some not so much.
I'll update this occasionally as kind of a sticky...