asterisk users - Jan 2006 - Echo while using Headset with Polycom IP 501 / 601 Asterisk 1.2.1

I'm hearing an echo when using a headset with my IP 501 / 601.  The
phones are using BR 3.1.2 and SIP 1.6.3.  I use tftp to configure the
phones.  The sip.cfg is the default from polycom except for the
parameters required to connect the phones to asterisk.

I have absolutly no echo with the handset, but do have a slight echo on
the speaker phone.  I haven't ruled out room acoustics as the source of
the echo on the speaker phone.  The headset is where the echo is most
noticeable, the delay is about 1/2 second.  The persons that I'm talking
to on the other end of the connection do not hear an echo.

I noticed in the sip.cfg that voice.aec.hs and voice.aec.hd are disabled
while voice.aec.hf is enabled.  Polycom Admin guide says don't mess with
the settings -- I'm thinking I will enable the hs, and hd and see if
that makes a difference.  Does anyone out there have experience with
tweaking the echo cancellation, and suppression settings in sip.cfg for
the polycom phones.  Any help would be appreciated.

--Stagg