i've noticed this, but not sure it's the exact same problem. but from what i've found, it's caused by your volume being too high, or the person you're talking to's volume being too high. When you talk, the volume is up so that their microphone is picking up your voice, and sending it back to you (delayed). This causes an echo. If you turn the volume down (or if they turn it down), it helps. The iphone's design for whatever reason makes this prone to happen.