There are quite a few threads, comments dealing with Silverlight applications for developers, but many of them lack information about webphone devices and background support related to SL. I found the 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. After testing Ozeki VoIP SIP SDK in Visual Studio 2010 I went on using it. It sounds a bit like sticking to this sdk, but I have really experienced that the sdk and support team is great: they have helped me significantly in my work.