Remote Assistance Invitation Missing Public IP Address

I am trying to use Remote Assistance (RA) over the internet between 2 WinXP SP2 computers. Each computer is behind a NAT gateway/router, one UPnP (Linksys WCG200ver2) and the other non-UPnP:

pcA <---> UPnP NAT gateway <---> Internet <---> non-UPnP NAT gateway <---> pcB

QUESTION: why is the RA invitation generated on pcA not including the Public IP address of the gateway, even though the it is a UPnP enabled gateway. Without it, the connection will fail.

DETAILS:
UPnP NAT gateway (pcA) port forwarding of port 3389 to private IP of pcA.
Remote Assistance is enabled/allowed on both pcs (computer>properties>remote).
RA is allowed in Windows firewall on each pc. Windows firewall is disabled on each pc.
Remote Assistance exception, checked
Remote Assistance-Windows Messenger and Voice, checked
UPnP Framework, checked
Internet Gateway Device Discovery and Control Client is enabled (add or remove programs>add/remove windows components>components list>networking services>details)
SSDP Discovery Service is running on each pc.
an Internet Connection icon appears in "Network Connections "
UPnP Device Host Service is running on each pc.
a Broadband Router icon appears in "My Network Places "
Norton Internet Security 2008 on pcA: local port 1900/UDP open for UPnP Discovery (Shared Networks); local port 2869 open for SSDP (Shared Networks)


EMAIL Remote Assistance: If I generate a RA invitation on pcA (UPnP), the public IP of the gateway is not inserted into the invitation. I can establish a RA connection by editing the invitation, inserting the public IP address, and emailing it to pcB. When the invitaion on pcB is executed, a connection is established. The reverse is also works, from pcB to pcA.

WINDOWS MESSENGER Remote Assistance: a RA connection cannot be made in either direction using Win Msgr. Does this fail because the Public IP is not being supplied? What else should I try to correct this?

I have made successful RA connections, using Win Msgr, with several other computers (other than pcB). Is this because they are behind UPnP NAT devices and their Public IP address is being supplied?

 

Port 3389 is forwarded on the router/gateway (1st line under "Details ").
The RA connection can be made successfully if I insert the Public IP of the router into the invitation. WinMsgr also works on with some computers. So, I believe the software firewall does allow the clients.

Is there anything else that should be checked to insure that the pcA gateway (UPnP) will insert the Public IP into the invitation, as it should if UPnP is working correctly?