I upgraded my iphone 6 with 8.3 with the well known problems of bad audio quality during some calls.
I downgraded to 8.2 and the problems disappeared, now I still have installed the 8.2 waiting some feedback about the new iOS release before the upgrade (now is no more possible to downgrade to iOS 8.2).
I think that the bug has been introduced due to the new call over LTE introduced in the iOS 8.3 firmware version.
I suppose that this new feature should be implemented by the mobile operator with an upgrade of their network system otherwise the iphone could setting the call with a sort of "hybrid" status with the result of the degraded quality during the call.
If I'm right, Apple could wait that the mobile operator will be ready to implementing the new feature in their networks.
I don't believe that Apple at the moment doesn't identified the problem and is not able to solve it with a patch, probably is because they are waiting for something..