asterisk users - Mar 2003 - Bad echo ATA -> Asterisk -> IAX -> S100U

I am running two asterisk servers on two separate (~15 hops apart) IP 
networks.

I have an ATA 186 on one end as the UA, and on the other an S100U.  I 
have an extension at either end that uses IAX to talk to the other system.

The overall sound quality of the S100U, as is pretty widely known, 
doesn't really sit at the table with the ATA186, but tonight I 
discovered something that almost makes the two of them useless when used 
together.

When a friend at the remote network picked up the instrument attached to 
the S100U and called me, I had a horrible horrible echo when talking to 
him on the ATA.

It was basically every sound I spoke coming back to me, clearly and 
completely, a fraction of a second later.

Is this a known situation, or have I stumbled on to something?  I do not 
have the aggressive canceller invoked. . .

Thx.

B.