Mine works around a bad design decision in my gate’s GSM intercom and stops endless missed deliveries.

The intercom does ring-around: a visitor presses the button, it calls the first number on its list (my mobile), and if nobody answers it tries the next (my wife's mobile). Except it counts voicemail as an answer. If the first phone goes to voicemail, the intercom decides the job is done and numbers two and three never ring. So the list is effectively one number long if my phone has no reception. I assume nobody tested it against a phone that was switched off or travelling on the London tube.

I gave up on the chain calling in a the first week of several failed delivery attempts. Now an ESP32 and a SIM7600 sit first on the list instead. The modem picks up after 3 seconds, sends a # DTMF tone (the intercom’s open command), hangs up, and texts me that someone came in.

Since this is a mass used car security gate it is a weak security control and more to hinder casual drivers parking where they are not supposed to - and now I get my packages too :P