For the past 3 years we have been using PBX In A Flash on an Atom-based PC. This resulted in massive savings, the typical monthly phone bill is < $10. Calls to Europe are less than $0.018/min.
But Asterisk has a few issues and I am generally afraid to upgrade beyond 1.4. So, I went looking for an alternative. The chosen one is FreeSWITCH. Even better, it will run on a Seagate Dockstar, which has no moving parts. (The CPU fan on our PBX likes to seize up once in a while, not nice to cook your cpu that way).
I purchased a Dockstar on eBay and am now about to place OpenWRT on it.
Some useful links:
More to come as I move forward.
FreeSWITCH now has a book describing its configuration and customization. Well worth the price.