Yes...I'm having the same problem as well.
• When I'm on the phone using the built-in earpiece, the other individual hears their own echo.
• On my end, I hear no echo and the call is clear.
• When using the speaker-phone feature the echo ceases.
• Phone volume levels during conversations were set at 10.
• I called several different phones on various carriers (including land lines) and all produced an echo.
• I checked the sim card and made sure it was clean and installed correctly
• By reducing the phone volume down to 6 during conversation, I was able to eliminate the echo for the most part. Above that level and echo returns.
• I believe that the mic near the headphone jack is picking up the conversation from the earpiece when the volume is set above 6, creating the echo on the other end. Again, never on the Nokia's end.
This phone has two mics. View this link:
http://www.nokia.com/us-en/support/product/lumia900/userguide/
I think that's the problem.