There are a couple of voips hanging around out there. Most of them take up too much computer power to be used while using another program, like a virtual game tabletop. http://www.strolen.com/forums/viewtopic.php?p=33876#33876
Someday, when these are pay services or when distributed processing allows for SIP connections without a SIP server, they will add VOIP directly to these products.
However, there are a couple of tools that are usable. Today. While using another program.
There is RogerWico, a product I used to sell long ago... and now they give away for free.http://rogerwilco.gamespy.com/
The others are pay products, go to Quake, Unreal, and Rainbow sites if you want to find them.