Module available here: www.elektor.com/door-spi-150400-91
Build your hi-tech doorbell with this GSM HAT board for the Raspberry Pi. You’ll know who rings at your door even while you are away from your home. The doorguard uses a M95 GSM module from Quectel, and is connected to a microphone, a camera and a pushbutton mounted at the front door.
When a visitor rings, the camera takes a picture of him/her and sends it in an MMS message to your cell phone. You can start a telephone conversation via the built-in intercom, and you can even send an SMS to open your front door.

View the origins of this project on the Elektor Labs website
 
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