I did something a lot simpler than this to have some more control: I gave a Twilio number to the building manager for the door box, then had an app where I coul

I did something a lot simpler than this to have some more control: I gave a Twilio number to the building manager for the door box, then had an app where I could give out codes to people. Valid codes responded with a "9" DTMF signal which opened the door, a "1" forwarded the call to my phone.