Access to our back garden is via conservatory. We are planning to move in the next year or so so don't want a flap in the double glazing. It couldn't go through walls as they are raised above garden level. Cats would need to go up 60-80 cm to access it. Prefer them to go out the back as the nearest road in that direction is much further. I work from home but often in a different room. Dgirl cat is fairly lazy and isn't out long. Dboy scratches at door and often I do hear him. I check regularly anyway. I was wondering whether I could train him to ring a bell. A sort of Pavlov's dogs in reverse.
Before anyone questions why we got cats if we can't put cat flap in. They adopted us as they were starving and wrapped everyone around their little paws!