Hi,
A simple technique is to adapt a door sensor which uses a magnet and a reed switch. Normally, closing a door moves a magnet on the door close to the sensor on the frame, magnetises two metal strips in a glass vial, which closes the circuit and triggers the sensor.
Inside a ‘ding-dong’ doorbell is a basic solenoid - press the button, the coil energises and a rod which hits two metal bars. The solenoid also produces a wider magnetic field, which is enough to reliably trigger a Z-Wave door sensor next to the coil.
I simply added a Z-Wave Fibaro FGK-10x inside the ding-dong doorbell and reconfigured the sensor to normally open.
To keep the sensor isolated from doorbell voltages you could wire a relay or even opto-coupler in parallel with a bell and the switch contacts to a door sensor. Most UK doorbells are 8-12v AC, which is not ideal to interface with 3.3v, 5v, or 12v sensor inputs.
A door intercom is a lot more complex, as it is likely to use 2 or 4 wires for the bell, audio and power. Reliably detecting a door bell push will need a lot more experimentation to work out what happens across which wires. I’ve seen systems with different number of doors or intercoms use very different circuitry (e.g. bpt, Fermax, etc).