Yes, I've had the same problem, and I couldn't find any information on voice chat alone.
After searching through MSN Messenger's Help, most all of the information I found was referring to Voice chat through Netmeeting. They did give detailed instructions on how to set that up behind a firewall, and most routers won't work (they have to be UPnP (universal plug and play) enabled---in other words, netmeeting requires dynamic port allocation, so that's why they need UPnP. Of course, Microsfot's ISA Server can do this).
After further searching, I could not find any information about Voice Connections only. I'm still searching.
FYI--the reason you can voice connect to your computers behind the lan is that they are on the same private LAN. When two computers on separate private lans attempt voice connect, they will try to connect to the private IP address. This won't work.
However, if you have one person on a private LAN, and another person on a public IP, ie, connected straight to your MODEM, or DSL Connection, you can connect only if the person on the public IP requests the voice connection. If the person on the private LAN does this, it won't work. I used to do talk to my sister this way in Georgia.
