My dog just pounds on the back door or barks until I let him in. However, I do not have a doorbell on my back door.
I’m not kidding, he pounds on the door.
We had a cat that would ‘knock’. We had a screen door and he would grab it by his paws and pull it out and then let it go. Sounded like a knock. Someone was at our house when he did it and they thought someone was knocking on the front door.