If you are using the e-smith as a gateway, I will guess (without beeing 200 % sure about it) that you will not be able to make a voice conversation via the MS Messenger. The reason for this is that Microsoft are using one spesial protokol uPNP (I believe the name is) that is not supported by traditional nat routers. I'm only using the e-smith in server mode so I have not really tested e-smith on this point, but beeing a "traditional Linux" I would not expect it to be able to make the "uPNP play".
I am using a Netopia hardwarerouter myself and it is not able to do the transportation of the uPNP protokol.
One way around this is not to use the yahoo messenger instead, that have basicly the same functions and that generally has a lot less problems with nat routers.
http://messenger.yahoo.comOne other solution will be to use some iptelephone program to transport the sound while using the ms messenger. Picophone do not have the same sound quality, it is actually rather poor, but it usually traverse nat routers and firewals very well, and it is very easy to use. It is very small and can also be stored on a floppy or on a memory stick if you want to bring with your free internettelephone while you are traveling.
http://www.vitez.it/picophone/