I have been using my doorbell automation for a year or so now..
So what I did was to hack the siemens intercom doorbell.
This device has a button for opening the door as well as a inbuilt speaker. So its an easy hack, to open the door connect a relay in parallel with the open door button on your device. To find out is someone is ringing the doorbell I used a 2usd mic shield for arduino hooked up in parallel with the speaker element on the intercom. This is then wired to my GPIO on rpi as an input.
So whenever I have guest over or lost my keys I just ring the doorbell in a specific order and my doors open.. Works brilliant 
Only thing I am still considering is to install a phone that is hooked up to the doorbell so if I am not home it would actually call my phone.. Now I just get push notifications, but I have no idea who was at the door..
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