Symptom was that the person on the other end of my calls heard their voice echo. I found that the problem only occurred with one of my headsets. No problem with a different headset and no problem when I had no headset connected.
After spending two days in various forums I found that I could stop the echo problem by turning off Advanced Calling.
SETTINGS=>ADVANCED CALLING=>ADVANCED CALLING=>HD VOICE (remove the green check).
