Just a quick update after a week:
The echo is definitely a problem only with incoming calls. If someone does call you and they're getting the echo, just toggle "hold" and "unhold." It should fix the issue. I've decided not to bring my device back to the ATT store and I'll stick it out, hoping this is an issue Microsoft could easily fix. It's interesting to see that some units have this issue, while others don't. Not sure how to explain that as I thought it might be firmware related.
The echo is definitely a problem only with incoming calls. If someone does call you and they're getting the echo, just toggle "hold" and "unhold." It should fix the issue. I've decided not to bring my device back to the ATT store and I'll stick it out, hoping this is an issue Microsoft could easily fix. It's interesting to see that some units have this issue, while others don't. Not sure how to explain that as I thought it might be firmware related.