The purpose of the "ECHO" is to prevent the other party from hearing himself (echo). If a user is using headsets, then there is no need for AEC since there will be no echo generated on his side. In this case it might be better to disable "ECHO" to avoid unnecessary audio filtering and also to save CPU. A user that is using hands-free mode, MUST have "ECHO" enabled to prevent all other participants from hearing themselves. If ""ECHO" is enabled but still echo could be, it means that the acoustic echo cancellation software that is running is not robust enough to handle the echo. Following is a nice blog series on the principles and challenges of acoustic echo cancellation.

The purpose of the "ECHO" is to prevent the other party from hearing himself (echo). If a user is using headsets, then there is no need for AEC since there will be no echo generated on his side. In this case it might be better to disable "ECHO" to avoid unnecessary audio filtering and also to save CPU. A user that is using hands-free mode, MUST have "ECHO" enabled to prevent all other participants from hearing themselves. If ""ECHO" is enabled but still echo could be, it means that the acoustic echo cancellation software that is running is not robust enough to handle the echo. Following is a nice blog series on the principles and challenges of acoustic echo cancellation.