Seeing AT commandes described as the "way that humans and machines can interact with a cellular modem" made me feel old because they didn't just say "remember AT commands from manually driving your Hayes 9600baud dial up modem?"
Edit: To clarify, before then you would have something like a 300 baud modem that didn't have an out-of-band command set, so you would dial the number on your phone and flick a switch on the modem to make it connect and everything would be sent to the other side verbatim. The "+++" is the escape sequence you send to a Hayes modem to break it into command mode, the "AT" is the prefix for the command (attention?), and "H" means "hang up". IIRC, puritans would say "ATH0" because the "0" represents some particular state to hang up to, but it's the same as "ATH".
IN#3 (Apple ][ command to hook input from slot #3)
^A ^F (modem command to set full duplex)
^A ^Q (modem command to pulse dial the following number)
^A ^Z (modem command to hang up)
https://en.wikipedia.org/wiki/Hayes_Microcomputer_Products#E...
https://www.computerhistory.org/collections/catalog/10264598...
https://www.ebay.com/itm/393165436881
Later on once the Hayes AT command set took over and BBSs became popular in the 80's, there was a flat $25/month subscription service called "PC Pursuit" that had banks of modems in different cities connected by a network, which you could dial up locally, then connect over the network to another modem, and dial out to a local BBS system. It was a subsidiary of Telenet, which spun off from BBN, and later was a subsidiary of U.S. Sprint.
http://www.bbsdocumentary.com/library/CONCEPTS/SERVICES/PCPU...
https://bbs.fandom.com/wiki/PC_Pursuit
https://blog.tmcnet.com/blog/tom-keating/voip/pc-pursuit---t...
https://en.wikipedia.org/wiki/Telenet#PC_Pursuit
There was a fun game you could play called "PC Roulette", in which you first connected to a remote modem with the PCP command, then issued the command "A/" directly to the modem, to redial the last number somebody else dialed on that modem!
It usually worked. I don’t remember if the baud rate was 300 or 110.