To troubleshoot an echo or feedback during a live recording session:
Mute each caller one by one to find out which is producing the echo.
The guilty party causing the echo will be clear because when you mute their line, the echo will stop.
Echoes are caused by the following factors:
Caller listening to live show on computer while dialed into your show. (#1 culprit)
Caller in a big empty room.
Caller using a laptop microphone to call into show.
Caller using a speakerphone to call into the show.
Once you find the caller that has the echo you can instruct them on the above and have them call back in or you can drop the caller altogether.