The iDoorCam smartphone app works with a camera mounted to a doorbell
When a visitor presses the bell, the camera begins filming their face
Images are sent wirelessly to the mobile phone so users can see who it is
The user can then see and hear the guest through the Android and iOS app
You can build one of these with the raspberry Pi, a cheap webcam and a few bits taken from an old computer keyboard. For about £35, everything is on the internet so you just need to follow instructions.
The bell’s mechanics should be recessed behind the door and just the button press showing.