Only if their modem didn’t implement the Hayes command set properly or you could otherwise control the per-character timing of the OS sending. It required a pause (1sec by default), “+++” with no pauses, another pause, _then_ the ATH command
I had an external USRobotics 56k modem, I was immune. But the many many "bulk" no-name modems were vulnerable. You could ping entire ranges of dial-up IPs and watch the results on big IRC channels. Uhmmm, allegedly :)
