asterisk users - Feb 2003 - ATA <->asterisk< -> IAX< -> S100U: Horrid echoes

I have an ATA box on my local LAN, talking to asterisk.  On a remote LAN 
I have another instance of asterisk; that box has an S100U as its 
primary instrument.

I set up an extension to dial between the two, sort of a "private
line,"
and with the exception noted below, things appear to work just fine.

That exception is echo.  The "ATA" side of the conversation has a 
horrible echo, which makes the call almost unusuable.  On the remote 
(S100U) side, the voice sounds normal, and the echo is not audible.

Does anyone know if there's a fix or a workaround, or what I might tweak 
in this case?

The S100U in general has fairly poor sound quality, but it is usable. 
The ATA works flawlessly in virtually every other scenario of use.

Thx.

B.