OK, so besides being the title of a Rush album, I titled this post to explain the reason behind echo when using VOIP.
Echo is basically caused by two things, Latency and Volume.
Latency is the delay from when your voice is actually picked up by the handset of your VOIP endpoint and when it is transmitted from the receiving end's endpoint to your ear.
Volume comes into play because if you have sufficient volume, the mouthpiece on the far end endpoint can pick up your voice from the Receiver and retransmit it back to your endpoint.
This happens all the time in the wide wide world of TDM, but you don't notice because the latency is so low, your brain filters it out. With VOIP Latency can be much higher, so lets say you have a quarter second of latency between you and the far end endpoint. Normal conversation will be perfectly fine and you won't notice, BUT, if you hear yourself, you are in essence doubling the latency (trip out plus trip back) so you hear yourself a half second later (very similar to yelling at a canyon and hearing the echo back.)This leads to people sounding mildly retarded, like when you hear goof balls sing the national anthem, and then they hear there voice three or four words into the song at a stadium.
The most effective solution for this is to PAD (or reduce volume) between the endpoints either one way, or both ways depending on if the echo is only on one side or both. It's not hard to do and with VERY LITTLE experimentation after finding the echo scenario you can correct your problems.
So now you know what causes echo and how to fix it. Can someone PLEASE call the guy who keeps calling us and asking us to fix his echo what I just told you. Apparently he either doesn't believe us.