I am developing a VoIP app for my Nokia Lumia 710. The phone runs on Windows Phone 7 OS. Since we know that there are lots of apps for Andorid and iOS, I belive that developing for Windows Phone 7 has lots of potentials. So while I was browsing on the Internet found a page with a detailed guide. It provided me a good start because there are code snippets included (if you are interested: http://www.voip-sip-sdk.com/p_511-wi...form-voip.html). Hope you will find the shared information useful and please note that I only wanted to give an advice.