asterisk users - Nov 2005 - Please help me diagnose/fix this echo

Just when I was ready to work on the deployment of several
hundred Asterisk SIP phones, I bumped into a serious
potential show stopper: an echo.

I went to Astricon and attended the Echo Session.
I also happen to know some (not a lot) about impedance
matching, etc.

My problem is mainly related to the DTMF digits when my
users call an IVR system, from a SIP phone to a PSTN IVR.

Would some of you folks out there (preferable knowledgeable)
dial into the Boston Verizon voice mail number:

1-617-491-0012

and then leave a message, for instance, in my voice mail:

617-576-2545

(you don't have to leave anything)  In fact, you will probably
not be able to leave any message because the DTMF digits
will not be recognized.

These are my settings:

Asterisk 1.2.0-beta2
Sipura 2100
Cisco AS-5300

A colleague called from his Linksys digital hardphone and
was able to leave me a message (i.e. the DTMF digits were
recognized by Verizon), but then when he tried from a
Sipura ATA he failed.

It looks like it is worth purchasing the whole telephone
(as opposed to a ATA+phone combination) if you want to
prevent echo.

I tried changing the "FXS Port Input Gain" and "FXS Port Output
Gain" from the -3 default to -2 and -1, but no luck.


TIA,

-Ramon F Herrera