Hello,
I have a Nokia Lumia 710 that runs on Windows Phone 7 OS. Both of them are great so I decided to develop an own app for it. As we know there are lots of apps for Android and iOS. Developing an own app for Windows Phone 7 has potentials in it. During the development I came across a page with a detailed guide about app development. There are code examples that I found really useful during the development process (If you are interested: Ozeki C# VoIP SDK - A SIP SDK for software developers). In my point of view it can be a good starting point for anyone.