I have recently purchased a Lumia 710 and I am really satisfied with it. The thing is that I would like to develop an own application for it. I know a little about programming but I am not a professional. This is the reason I decided to use an SDK for development purposes. After testing several SDKs I think there is one that is the right one for this task. There is a page that provides information about the development and has code examples (if you are interested Google on: voip-sip-sdk/page511 "Windows Phone 7 Development"). I could benefit from the information provided by the page so that is why I decided to share this information. This is only a suggestion and hope you will find it useful.