We have a fully working doorbell. I really can see how you can miss it. It's about 4'6" off the ground on the lock side of the door - exactly where you would expect to find it.
Luckily, I work in a front room, facing the street, so I can see people coming up the path. That way, I know they are there, because for some reason very few people seem to know what a doorbell is for. I've just had a guy pathetically tapping the glass which seems to be what they must teach nowadays.
I must ask the next numpty why they didn't use the doorbell.