Like the presence button but where we would call/send SMS to a phone number to know if the hackerspace is open or not. Could also handle intercom to the doorbell on the exterior portal as a second step.
What would be said when calling the phone (test it online):
Leh hackerspace ay fairmay. The hackerspace is closed.
Leh hackerspace ayt oovayr. The hackerspace is open.
Another idea (second step) would be to set for how long the hackerspace is opened (pick up the phone, enter some menu − voice server-like interface would describe how to do it − with the rotary dial and set a delay or a closing hour, to be posted on Matrix or something).
See also this beautiful "rotary cellphone" featured on minimachines.net (in French). It uses an ATmega 2560V with an Adafruit Fona 3G board instead of the ESP32+SIM800L that we plan to use though, but it could be a nice inspiration.