There are quite a few threads, comments dealing with silverlight applications for developers, but many of them lack important information about webphone devices and background support related to SL. I found the http://www.voip-sip-sdk.com/p_85-web-to-web-video-calls-silverlight-voip.html link and think it's instructive enough to start a business application with webphone based on an sdk. After testing Ozeki VoIP SIP SDK in Visual Studio 2010, but I think in VB it can also be done, I went on using it. It sounds a bit like sticking to this sdk, but I have experinced that the support team is really good: they have helped me significantly in my work.