My house also came with an existing NVR camera network which I can view in home assistant over my router without it ever going to the cloud as well.
I have a Wyze camera and their janky HA integration seems to have stopped working after a firmware update. They're also the epitome of enshittification and want to nickel and dime me for every feature -- I'd be glad to ditch them.
Until one day they auto-update ...
IME you have no control over the baseband chip of a cell phone, no reason to trust it's not enabling its camera or microphone at any time. I have a flip phone which comes in a non-camera version. I have an iphone without a SIM I can connect to its hotspot if I need to do something smartphoney.
How do you handle smartphone cameras?
1. The doorbell cam is connected only to the LAN.
2. The doorbell cam is definitely blocked from accessing the internet.
3. Having access to the LAN from your phone through VPN allows you to watch the feed and talk to people at the door through the app.
2. I have not gone through the trouble of isolating them on their own VLAN, but I don't see any traffic coming from the devices. This is something you will want to audit on your own network if it is important to you.
3. I have viewed the camera output through VPN, but have not yet tried speaking through the doorbell (or through one of my camera's audio output). I don't see why it would not work.
The Reolink doorbell camera is a typical recommendation and can work fully locally and supports well the common standards like RTSP. 2 way audio works as well.
I believe the NVR Reolink sells can also work fully locally as well. Which also can integrate to home assistant.
Relatively low tech compared to somehow hooking up a camera livestream system to ring your phone via the internet in some way but it works