Hello,
I have gone through the provided information and made good use of it. I own a Lumia 710 and the information you shared was really valuable, thanks. I have also tried another mehtod (according to the followings: http://www.voip-sip-sdk.com/p_511-windows-phone-platform-voip.html) that was also really good. If you want you can also have a look at it. Hope my information will be as useful as yours was.