There were a bunch of different mechanisms depending on vendor. Some required a specific time interval in between the + signs, so that typing very slowly, or instantly, wouldn't trigger the escape.
Some vendors had bugs in which when +++ appeared as part of the incoming data it would also trigger the firmware escape.
And of course programmable terminals had hilarious exploits where you'd get someone to run your program, it would program the terminal to emit a macro when you hit enter, the macro would include +++ATZ, etc., etc.
My favorite modem story was when I worked for a company that shipped auto parts warehouse management systems. They would have several SCO unix boxes with serial port concentrator cards and port concentrators on those concentrators, such that you'd have 256 modems on a single box and RS-232 cables everywhere. Frequently the operators at the auto parts warehouses couldn't be bothered to do cable management, so at one facility, the entire floor of the server closet was carpeted literally 2 feet thick with a random mess of RS-232 cables, each one leading to a modem. When standing on the cables killed a modem or two, they would just string a new wire rather than try to fish out the old one. Kind of like Google's "let it sit in the rack" policy for failed servers, but writ extremely small and pathologically badly.
The entire rack gets replaced with new equipment eventually. Probably get replaced every few years or so.
For most of the SMB clients I have, I don't build new servers any longer. I can buy rackmount servers pulled out of a data center that are a few years old at about 1/3 the price. I just put new disks in them and a new OS.
Some vendors had bugs in which when +++ appeared as part of the incoming data it would also trigger the firmware escape.
And of course programmable terminals had hilarious exploits where you'd get someone to run your program, it would program the terminal to emit a macro when you hit enter, the macro would include +++ATZ, etc., etc.
My favorite modem story was when I worked for a company that shipped auto parts warehouse management systems. They would have several SCO unix boxes with serial port concentrator cards and port concentrators on those concentrators, such that you'd have 256 modems on a single box and RS-232 cables everywhere. Frequently the operators at the auto parts warehouses couldn't be bothered to do cable management, so at one facility, the entire floor of the server closet was carpeted literally 2 feet thick with a random mess of RS-232 cables, each one leading to a modem. When standing on the cables killed a modem or two, they would just string a new wire rather than try to fish out the old one. Kind of like Google's "let it sit in the rack" policy for failed servers, but writ extremely small and pathologically badly.