If you are experiencing an echo, the most likely culprit is a microphone and speaker that are too close together. Optimal audio will be heard when using a headset or earbuds instead of the built-in computer speaker which is located near the built-in microphone.
Additionally, audio could be coming form multiple sources. If you joined the meeting via computer with a mic, you do not need to call in via telephone. Also, make sure your computer isn't picking up audio from someone else's machine. If you are in the same room with another meeting participant also using speakers, use audio from only one of the computers.
As a conference call host you can quickly eliminate audio interference coming from a meeting attendee by pressing 1* to mute all meeting participants. Use the Conference Manager to identify the source of the noise and either mute all participants, mute the bad line or request the participant to mute their line.
Keep in mind that if you or a meeting attendee are joining the conference by mobile phone the issue may be poor cell coverage. Try to join via mobile where there is good cell service.