I am a fellow .Net developer and I had a task to build a Windows PBX. I just want to share a good site with you that gives good tips about how to develop a Windows PBX. There are also other useful things provided there: http://www.windowspbxguru.com/
Anyway, I only want to give a little guidance about this topic and the above site helped me a lot.
I only added this post so others could also gain benefit from it and not with any advertising purpose.