Very interesting. I have tried VoIP calls on three different wireless routers, all Linksys. One of my routers is brand new and I'm in the process of returning it because of some very strange behavior. This router is one of the new wireless-N (802.11N) routers and when connected to it with an ethernet cable I can reach any web site but when connected to it wirelessly, I can get to some web sites (such as CNN) but not to others (like Google). It was my discovery of this problem a few days ago that prompted me to try VoIP calls on two other routers. However, both of these routers were also Linksys (both wireless-G routers).
Another funny thing about my E60, and I'm not certain if it's by design or a bug, but I cannot connect to my wireless LAN unless some application needs to connect. Normally, one would establish a network connection and then launch a browser or email client. However, with the E60, in order to connect to my wireless LAN, I have to launch my browser, which then initiates a network connection. If I then close the browser, the network connection is terminated. I take it from your post that you can initiate only a network connection?
I'll keep working on this and post anything I learn to this forum and topic.
Mark