I finally found the cause of the echo I was hearing of myself, when talking to my wife. I had at first thought her volume was turned too high. Here's the cause. She was moving her phone higher back toward her ear. This caused the mic to be pressed against her cheek. At least partially, if not totally. If the mic is covered by the person you're talking to, you'll hear your echo. So many of these issues could be that. If you hear the echo, ask the other person to be sure their phone's mic isn't pressed against their cheek, or covered some other way.