Hayes figured out how to make that impossible: The "guard time" interval. When you want to interact with the modem, you have to send _nothing_, not a single character, for a whole second before sending +++ and then another whole second of nothing. Only then would the modem place you into command mode. That way, +++ could happily appear in payload but never mess with the modem, because there'd be other payload either side of it.

Of course they patented this, and anyone who licensed the patent was likewise immune, but the manufacturers who didn't, could easily be plonked like this.

My first "hack" involved a BBS scripting language that involved a delay command... ;)