You have the RCD-510 (non-navigation unit).
Never had a problem with my bluetooth calling or volume of my calls. I'm using Android (originally a G2, now a Galaxy S III). To me, it sounds like it could be 1 of 2 problems. The first could be the phone, and the second could be your bluetooth module. I was test this by having someone with another phone (probably completely different type of phone and service) connect to it and see how it sounds. If it sounds the same, I think the bluetooth module is faulty.
EDIT: Actually, thinking about it some more, if the call goes through, just after the message and a few seconds of waiting, I would probably think it might be the phone not supporting the car bluetooth (or vice versa). There hasn't been any software changes for the bluetooth that I know off, so def try to test this with another phone.