Years ago, I hooked up my apartment buzzer to Yo with a raspberry pi. https://github.com/sambostock/yoorbell
It was super insecure, but if you yo'd it, it would let you in, and if someone buzzed the buzzer, it would yo me. Exactly as much info as needed.
Ooh I did a similar thing at in college where I used a raspberry pie to hook up the door handle to speakers to play “The Boys are Back in Town” whenever someone opened up our front door at the beginning of the semester.