I have a habit of putting my iPhone in my pocket upside down. This apparently has, over a few months time, caused lint and debris to collect in the earphone jack. I've jammed my buds in a few times so it was good and packed in. It occurred to me that the earphone jack is rigged to take a microphone input from the iPhone ear buds. I figured if there was fouling in the socket that it might be causing the problem. (Possibly a short across the microphone and earphone connections.) Sure enough after some careful surgery with a paper clip, and some compressed air, I cleared the impacted debris. Calls are now crystal clear!