I live in a rental apartment. We have two bells, one is for the main door (intercom), and another is for apartment doors (classic doorbell). I can’t touch any wires there, so I don’t know what are my options.
I try with a vibration sensor, but that didn’t help for any of them.
I have a camera in the hallway that can capture sound and send that with RTSP audio stream.
What are my options there? Is there any?
take a look at this: https://iotassistant.io/esp32/smart-door-bell-noise-meter-using-fft-esp32/
I have managed to get that code to run on a esphome esp32 and I can pick up my doorbell and my intercom but I’m still getting false positives now and then, might work better if I had it closer to the door, for now its 3m from the door and I have set it up so I only get notifications when I’m not home.
if you are interested I can try and clean the code I use up a bit?, or try the original above.
It took me a while to find it again, but i remember someone posted it somewhere …using a reed switch 
All you have to do is get a standard door sensor up against the coil. When the doorbell switch is pressed, it energizes the coil creating a magnetic field. That magnetic field activates any door sensor that is close enough.
You could also set a KY-038 with a esp32 or esp8266 near the buzzer from the bell. No need for physical contact with wiring.
https://community.home-assistant.io/t/dumb-doorbell-come-smart-by-a-sound-technique-weirding-way/264627