Okay, so over COVID we had a hardware failure in the PBX and I had to start from scratch with nothing but the wiring I had already completed.
The space is now wired with a Panasonic KX-TD816 Hybrid system. Every room is now equipped with at least one phone and each phone is assigned a three digit number (EG: the fab shop is 101), so calls can be made between rooms. Likewise you can now use any phone in the building to operate the PA system. The code is 32*.Full documentation how to operate the phones can be found and downloaded in PDF here - http://phone.manualsonline.com/manuals/mfg/panasonic/kx_t7230.html
Complete system documentation, wiring diagrams, configuration software and button templates have been given to Nicholas Adams. I'm keeping a copy myself, in case of emergencies.
The system is not fully utilized at this time. Additional lines and options can still be added at any time and yes, the system supports doorphones without any weird hacks this time. The door wiring is already complete. We just need to order a Panasonic KD-TD160 or KX-TD161. Over the summer I also purchased and delivered to the space additional electronic door strikes.