asterisk users - Aug 2003 - Problem with SIP Native Bridging and UPnP

My configuration is comprised of two Snom 200 phones, two FXO cards
connected to two PSTN lines, and one SIP account at iConnect.  Snom1 has
a VPN connection to the remote Asterisk server. Snom2 is using UPnP
behind a Linksys WRT54G router/firewall to connect to the same server.
All outgoing calls are routed to iConnect.

Snom1 works correctly. Snom2 can call Snom1, and receive calls from the
PSTN, but calls made to the PSTN through iConnect complete but nothing
can be heard. The debug output shows that Asterisk is attempting a
native bridge between iConnect and Snom2 at the time the call is
answered and the line goes dead. It would seem that a native bridge is
not working between iConnect and the phone behind a UPnP firewall.

Is the UPnP device the problem? Can it be configured to fix the problem?
If not, can native bridging be disabled for this phone? Otherwise, it
would seem that I will have to establish a VPN to this phone just to
solve this problem.