Sorry, I should have been more clear. The modem could also guard the sequence by requiring interstitial pauses, but I believe this was patented by Hayes in the 80s.
Indeed it was, as the referenced Wikipedia article notes; Hayes charged $1/unit for a patent license. As soon as the primary application of modems became Internet access, IP encapsulation protocols like PPP could have worked around the problem, but, AFAIK, never did.
