Have been thinking about this for a couple years. I have old phones kicking around. Battery shot, hardware dated, but the camera(s) and mic and antennas still work. Would be cool if there were a way to set them up (powered) to stream audio/video or even take stills at intervals (or motion-activated) and then sync the content to the rest of the devices on my network.
I don’t know how complex the programming for something like this would be. But I suspect it’s trivial for those who do know.
I use IP Webcam on my old phones as CCTV cameras, one for the inside of my garage and the other in my 3D printer enclosure.
Both cameras Just Work as mjpg cameras in Home Assistant. No internet, nothing.
What I haven’t (yet) tried doing is configuring them in my Frigate nvr (running on docker) - that might give you the complete package you’re looking for.