Just another idea… not sure how relevant to two-factor, but… when the safe is unlocked, you enter your phone number into the keypad, and it texts you a special random pin via Twilio, which once you enter it, your phone number becomes the first part of the authentication. Once locked, you need to enter your phone number, and then a new random pin is generated and expected to unlock the safe. Once it’s unlocked, a new phone number can be entered and the process started again. There could be a master code, for someone like a Hotel that would have to reset these daily.