When attending a meeting from a video endpoint (computer, lync, room system, etc.), use the audio portion of that endpoint to talk and listen, instead of dialing into the meeting from a telephone.
The redundant telephone connection can easily lead to disruptive echo (or audio feedback). For example, sound coming out of your laptop is picked up by the telephone microphone, causing echo.
Similar issue if multiple computers/room system endpoints are in the same room. Only one device should used for speaker and mic (all others should be muted).