[MarkusWandel]

+++ATH in the data stream did not close the connection unless the modem was really stupid, or misconfigured. The actual protocol was:

<pause>+++<pause>ATH<CR>

The pauses, I think 1 second by default, were exactly for this reason. "+++ATH" racing by in a data stream would be transmitted as is.

[schoen]

I think Hayes had a patent on requiring the pause, so competitors' modems sometimes didn't enforce the pause aspect of the design?

Or is that an urban legend of some sort?

[alexjm]

Yes, you're looking for US patent 4,549,302:

> Thus, even if the file being transmitted [...] includes occurrences of the escape command string of bits, it is extremely unlikely that any random occurrence of the escape command would occur unintentionally in the environment of the entire escape sequence, that is, the escape command string surrounded by a second of no data on either side.