In my house I have an audio intercom (Busch Jäger / ABB 83205 ) that has a four-pin-connector to the handset (speaker & mic), a buzzer and one important switch (open the door). In addition, there is an ONVIF-compliant Reolink Camera.
Ideally, when the buzzer gets a signal, on my mobile (or a local tablet) the video stream from the camera is visible and I get the options to a) talk through the intercom and b) open the door = closing the contact that is normally done through the hardware buttons.
Any advice would be great, until now I have not purchased any hardware so I am free to start from scratch.