If your conference call is experiencing an echo, there can be a few reasons how this can happen. All it takes is a single caller to introduce an echo with the following possibilities if a caller is:
Using a speakerphone where their volume is too high and the mic is picking up the sound.
Even a headset with the volume being too high can cause the mic to pick up what's being heard.
Experiencing a bad connection
How to identify the culprit and to resolve it:
As the host, mute the entire conference using 1*. If you as the host speak and do not hear an echo, you’ll know that it's coming from one of the connected callers.
Ask every caller to self-mute using 4* when not needing to speak. Unmute with 4* when they do need to speak.
Keep in mind that the host line could be a possibility, if the steps above did not identify where the echo is coming from, the host can self-mute with 4*.
You can end the entire conference to force everyone to rejoin in hopes of clearing up their call connection by using 78* from your host line then pressing 1 to confirm.
Note as the host, you have access to the Call Manager where you can manage the call and individual callers speaking rights from within your call. See more on the Call Manager.