> This had a major denial of service security implication in that it would lead to the modem hanging up the connection should the computer ever try to transmit the byte sequence +++ATH0 in data mode.

Does this mean you couldn't send the documentation for the modem using the modem?

No. You had to wait a moment after sending +++ to get the OK prompt, then you could send ATH0 then CR.

Blasting +++ATH0 (CR) would have done nothing, AFAICR.