You can try to create a situation as mentioned somewhere else in the forum to smarten a dumb doorbell.
That means that you put a relay between the two power lines from the fixed phone that triggers a door sensor or something like that whenever it rings.
But be careful, i believe there is some 40+ volts on the wires when it rings. So the electronics or relay has to be able to use that kind of voltage.
Found it. It’s in German, but the pictures should explain a lot.
