Both my wife and I have Samsung Galaxy S6 phones. This past week we were both upgraded to Advanced Calling 2.0. After the upgrade every inbound call has a 1 or 2 second delay before the caller can hear us. My father also has the Galaxy S6 and he is experiencing the same issue. If we disable Advanced Calling the issue goes away.
Also we have enabled Wifi Calling so I have not tested disabling WiFi calling to see if this resolves the issue.