Well, mine used to be about every other call, or 50% with no audio in the ear piece. I have not used bluetooth with this phone yet, nor the corded headset.
Lately though, I have noticed it not happening hardly at all. I just did 10 test calls to my office phone, and every one of them had audio in the ear piece. The interesting thing I found out yesterday, is that when it seems to be working almost 100%, if I kill the "Phone" process, and then launch the phone process again, I almost always have no audio on the first call I make, 100%. I just tried it on the 11th call I made to my office phone, and sure enough, after killing the phone process, and launching it again, the next call had no audio.
I had an appointment for Apple to call me, they took down all the issues I was having, as well as asking me what I tried, but eventually just said I needed an appointment to take it to the Genius Bar. I had that setup for Wednesday evening, but have decided to wait a little longer to see if apple addresses this issue in the next week or two. If it keeps up, and never gets addressed, then I will most likely make another appointment to take it in to try and swap it for another one, but want to make darn sure it's not doing this when I leave.
I'm hoping that killing the phone process is repeatable, which would provide a good way of testing before leaving the store. Maybe others can chime in here, and see if you have the same results.