This thread might give some ideas.
http://forum.voxalot.com/voxalot-sup...html#post10421
In some cases, echo can also be caused by the equipment being used by the remote party, so there may not be much you can do about it.
Some interesting technical articles on echo cancellation :
Causes of Echo - voip-info.org
CSD October 1999 Feature: Echo Cancellation for VoIP
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