From my own experience, if the top sound cancelling mic is blocked, it will make your outgoing call quality very bad. I had a TPU cover with no coutout for the tiny holes for the mic (top of phone to the right of the headphone jack). I had to cut a hole in my TPU protective case to clean up my issues. I just ordered an official Nokia case that has the cutout so it will look better than my modified case. Another thing you can try is to turn on the speakerphone. If you still do not get you voice to go out, it could be connection or carrier issues. If it works fine on speaker but not on normal talking, it could be related to one of the mics in the phone.