No Voice in Windows Messenger 5.0

-----Original Message-----
I am trying to use Windows Messenger 5.0 for voice
contact to another XP machine over the internet.
Initiating voice contact from either end fails with the
following message: "The network or computer you are
trying to reach is experiencing a problem. Please try
again later." Both PCs are behind Netgear DG824M
modem/routers which have UPnP enabled & the latest
firmware. Both PCs are XP and have all system updates
installed. There's no firewall s/w running. SSDP
Discovery Service & UPnP Device Host services are running
on both PCs. Windows Messenger between the two PCs works
fine for text. Remote Assistance from an XP Pro PC on the
network here works fine into the remote PC using Windows
Messenger. Messenger Voice works from this PC to the
same local XP Pro PC. I've run out of ideas...
.