If you've been in a conference call before, this has no doubt happened to you. Fortunately, there is an easy solution to this common problem: turn your volume down!
Unwanted echo or feedback often happens when one participant's volume is too loud, and it's feeding back into the microphone. If adjusting the volume doesn't help, insist that all conference guests mute their microphones when they aren't talking. It's a bit of a hassle, but so is listening to that horrible echo.