It looks like OpenSource is coming to the Windows PBX world! I was notified that FreeSWITCH has a native Windows edition. I haven't installed or played with it but take a look at the above screenshot and investigate.
I guess my opinion is about Free phonesystems is tempered by past experience but here are some of my thoughts about phone system, free and small business:
-the actual phone system is a small part of the cost of a solution.
-the handsets, gateways and implementation is the bulk of the cost.
-for a 20 user system a commercial grade Windows PBX is around $2000 and you'll likely fritter $2000 trying to figure out the free--so is opensource worth it for small business?
I'm not sure. But if you want to take a look, browse over to: