Here is an informal list of Windows based phone systems that I came up with in my internet research. They are in no particular order and I have not tested each one. I have not even installed some of these so this is in no way a recommendation of these products. In the interest of transparency our consultancy recommends, sells, implements & supports pbxnsip and sells & supports 3CX.
Axon Virtual PBX
Open Source Products
If you know about another Windows based phone system product I would be glad to here about it. Also, if you have any feedback on any of the listed products, once again, feedback in the form of comments would be very welcome!