Has anyone ever used SME Sever? http://wiki.contribs.org? Looks similar to NethServer, not as up to date of an interface it appears but I have used it at locations and it works well. It is based on CentOS.
For those stumbling in here down the road, the current consensus is that FreePBX is the better option for your new PBX. Elastix 4 did release, but is not being updated enough to remain a contender at this point.