There are three possible reasons for echoes

1. Someone has both computer audio and telephone audio active at the same time. Turn one of them off.
2. People using audio are sitting too close together. Ask them to mute their audio or move apart. 
3. Multiple computers with active audio are in the same room. 
   1. Some smartboards have their own audio.
   2. Ask attendees to mute their devices, move to a different room, or move apart.