can't help you directly since I have the G4 and Zumo 550 setup, but I've had no issues like you describe. Is your phone paired to the Zumo, then the Zumo to the headset? That's the order of pairing (the phone is not paired directly to the headset) in order to have all function properly.
There is possibly someone that can help you here....
http://www.zumoforums.comit's a free forum and lots of help available