On some phone calls on the iPhone 4, my voice echoes back at me making it difficult to carry on a conversation. This doesn't happen with every call, and sometimes the echo resolves during the call; Other times it doesn't. I have tried performing a soft reset, resetting the Phone application, and AT&T has also performed some sort of over-air reset. I exchanged for a new phone, and the problem persists. The phone is not in a case, but I am not holding it in a known problem area. I am running IOS 4.0.1 and have 4 or 5 bars of reception when the problem occurs. Anyone else having this problem or have ideas or thoughts on how to resolve?