asterisk users - Jan 2005 - Echo on SIP -- not on analog.

Okay, I'm stumped.  When I call the PSTN (through POTS lines), my analog 
phone phone works fine.  My SIP phones -- a Grandstream and a Polycom -- 
have major echo; roughly a .25 second delay.  Eventually, it goes away, 
which I guess is echo cancellation in action.  But, dammit, why does my 
analog phone work fine?  I've tried myriad CODECs, and various echo 
cancellation settings, to no avail.  Can anyone give me an idea of what 
I should be checking?  [Note that internal calls work fine.]

Thanks,

-Ken

Echo will only happen when you go from digital to analog.

Here is more info on this :
http://www.voip-info.org/wiki-Asterisk+echo+cancellation
http://www.voip-info.org/wiki-Asterisk+echo+analog+lines
http://www.voip-info.org/wiki-Asterisk+Echo+Avoidance

And remember : Google is your friend