I'm in a conference and there's a big echo when using the "Talk" feature:

1) The easiest way to prevent echoes when using the "Talk" feature, is to disable the "Auto" (hands-free) option located to the left of the "Talk" button.

2) Only use the "Auto" feature is you have head sets.  Standard microphones will pick up the audio signals from the speakers and re-send these signals into the conference room, creating an endless loop of echoes.

3) Watch the "Configuring your VOIP Settings Video"