That is neat, so how did you plan to send audio back to emulate a full intercom?
Is that something done easily with the KNX stuff? Being in Canada, Ive never really looked at KNX...
Ignore the KNX stuff, it's a red herring.
I figured to need 2 cat6 cables. 1 for the PoE supply to the camera, which returns video (and I assume audio from the camera board, although I've not tested this) as a video stream to browser (over the built in webserver). Then using the other cat6 (or cat5 should work) use two pairs for two push to make switches and the other two pairs as a return speaker cable to the door intercom. I've tested this and it works okay. At the return end (in my electrical cabinet in my case) I'd need two inputs to sense the push to make (this is where I would use some spare KNX inputs I have) and an amplified audio source for the door intercom speaker (I've yet to source a small suitable speaker).
I haven't finalised how to tie it all together at the software end, but the hardware is relatively simple.