I'm having similar issue. my iphone 4 syncs with the 550 and I can dial and place the call on the zumo. I hear the phone ringing in my helmet (Starcom1 Advanced) but as soon as the phone is answered on the other end the phone call is dropped by the zumo but the call is still going on to the phone alone. I can talk and hear it through the iphone only at that point. I'm running the current and most up to date firmware on both the 550 and iphone 4.