Cool. I have never worked on something like that but I did build and manage a Asterisk PBX with POTS lines a long time ago. It was interesting and we bought a bunch of cisco voip phones for cheap off Ebay. All-in-all. Probably spent around 5k for a system that a commercial installer would have charged around 60k just to install and a few thousand a month to "maintain". All of the features, call forwarding, DND modes, chat integration, Custom ringers for which line was called, call-tree for "press X to X". This was all in around 2010 and everything was Opensource. We tried to get fax over voip working at one point too (had some voip 'lines' for backup) That was a fucking nightmare back then. I bet it mostly works today though. Rather, I ended up building a hylafax system with a couple of old as hell modems that were hooked to POTS lines in the datacenter. The look on their face when we asked for a regular phone line to to colo rack was funny.