Hi,
I build Asterisk PBXs and also provide Asterisk/VOIP connections since 1998. I have built wholesale, retail and PBXs. I have live PBXs and FreePBX and custom call centers, etc in Canada and other parts.
I am not in this for money.... but would like to help you and get paid a bit for this....
Here is some FREE advice, check the codec, the internet connection or PSTN line connected to your PBX and your echo cancellation.
Another are might be the codec translation and you hardware may be TOO SLOW. If you are running X-WINDOWS/KD or while runnning FREEPBX/Asterisk then you are asking for trouble.
If my advice is not sufficient and if you have a budget between $250 to $500, I can help to fix or indetify the problem... but I think the above is one of the problems....
Good luck.
Warm regards,
Anthonus