asterisk users - Jan 2005 - Sounds cut out problem - HFC-S card, zaphfc, Xlite

Hello Asteriskians!

I have an Asterisk box with a simple HFC card in it and a bunch of
people using the Xlite software to connect. The HFC card is connected to
an internal extension on our legacy PBX.

So far so good. The Xlite clients can call each other, and the internal
extensions on the PBX and the Xlites can call each other, no problem.

The problem is when using an Xlite to dial an external number through
the legacy PBX.
What seems to happen is that there is some kind of noise suppression so
that unless the remote party is speaking very loudly the sound cuts out.

Now, I don't know if it is the ISDN connection, Asterisk or the Xlite
client that is causing the problem. I've tried different settings on
everything I can think of and trawled the web for days but so far
nothing useful. I've turned off silence suppression on all the Xlites.
I've turned up the rxgain on the ISDN channel in case it is too quiet. 

Nothing so far has helped.

Calling directly through the PBX from a normal extension phone doesn't
seem to have any problems.

Anyone have any idea what I should look at?

Thanks

Rob Scott
EPAM Systems Ltd.