Overview
As mentioned in my last blog post, I've also been looking at the panasonic DPT ("Digital Proprietary Telephone", also known as the "Digital Super Hybrid System")
This is a digital signalling format, with a lot of similarities to an ISDN BRI.
Audio is transmitted as G.711 (u-law or a-law, depending on the PBX setting). Line Power can be either +15V or +40V depending on the type of phone. The D channel somewhat resembles HLDC, but the message structure doesn't quite match.
I've written some notes on the protocol documentation here: DPT Protocol Documentation
Unlike the APT signalling, the DPT signalling would probably be quite complex to implement in a microcontroller, but it's fairly easy to implement in an FPGA.
I've attached some very early proof-of-concept RTL to this post (note that this RTL will likely fail for phones connected via long cables - the synchronisation logic does not properly lock to the start of the frame returning from the phone, but instead assumes a fixed time from the end of the frame from the PBX).
It's not compilable as-is (I've not yet worked out the best way of archiving FPGA projects that's easily reusable), but it might be a helpful starting point.
There's also some proof-of-concept C code as well, for managing some of the higher level protocol. Again, this won't be compilable as-is, but might be a useful starting point.