I recently arrived in Switzerland and faced the problem of notifying my friend when to pick me up from the train station. Had I known Switzerland still has payphones, I would have dropped 50 cents in one and sent an SMS. But imagine landing in a country without payphones, without internet, and with no working SIM card. How could you inform a friend what time you may arrive? A little signal sent on a 2-way FM radio could work. I came up with an even cooler idea. Payment networks are the most prevalent means of communication. You could set up a program that keeps checking your bank account for a transaction. Then, make a cash withdrawal or purchase just before boarding the train. The program could detect the transaction and send an email to indicate which train you’re on.
Back in the nineties, we could send prearranged messages free of charge by using payphones to ring other phones. Or we could make collect calls and encode the phone number to call back using dictionaries of names. Of course, then you could end up placing a collect call as someone unlikely such as “Tanya Rosenberg van Kent”