In two months, I've had one call that started off with myself echoing, and it went away after a few seconds.
I'm sure this is a network issue, and as we all know, the network can vary from time to time and place to place. If it doesn't go away, try calling AT&T to make sure all of your network settings are correct, and to be sure that you received all current over-air updates for your device. You'd be amazed at how "squirrely" your phone can act if it's not up-to-date with over-air updates from service provider.
Good luck 😉