Echo in telephony is a phenomenon that when a person hears what he has just said milliseconds after saying it. This could be confusing and quite annoying to both parties talking. This phenomenon usually happens on VoIP users when network's configuration isn't balanced and mostly on long distances.
In earlier international calls Echo suppression is used where turns off transmission when a person stops talking.
More information on the link below.
Link: Causes of Echo in Asterisk (and Telephony in General)
