EDIT: My original quest has turned into a fling with FusionPBX. If you are interested in giving it a go message me for help or join #fusionpbx on freenode. Moved 2 businesses offer so far its all pros and no cons. I have still reached out to multiple people about the original subject to see if we can get anyone else to release a FOSS competitor.
Based on some of the comments in this thread I am thinking about reaching out to developers of various projects to elicit a release of a FOSS version of a their stand alone PBX product that we could throw community support behind.
FreePBX currently is the most recommended small PBX solution on the market, FOSS or not. But as @JaredBusch mentioned in the aforementioned thread it still isn't a clear FOSS product. Everyone is interested in a production level system that is "cloud ready".
I thought I would outline a list of requirements, and I can add to the list based on anyone's feedback to this thread. Once I have enough feedback I am going to start to reach out to vendors/developers referencing back to this thread.
Basic PBX Functionality (Voicemail, Extensions, Ring Groups, Dial Plan, Call Forwards, Users, Presence, BLF)
Share line appearance (Something FreePBX doesn't mimic or support)
Built in Security/Firewall or SBC (border controller)
Simple installation process primarily for a hosted environment
Some type of clear/simple provisioning template system
No limits on users, devices, trunks, etc. In other words a true FOSS solution with feature parity to FreePBX
This could be the combination of several projects or a single product modified for a simple single tenant installation. Surely someone out there will support this as it could lead to exposure for their other products and services.