A good chunk of the IP cameras have user-exposed pins that can send events when the circuit is closed, as well as detect if a circuit is open (NC/NO circuits).

A good chunk of the IP cameras have user-exposed pins that can send events when the circuit is closed, as well as detect if a circuit is open (NC/NO circuits). IIRC they are intended to detect if a door is open or closed. I'm not sure if they are polling those contacts fast enough for 'normal' doorbell use, but it's there ...