[ralferoo]

First I've heard of ATHD command. Pretty sure my modem would ignore anything after ATH, and if you did ATDT would fail without a dial tone. But obviously, you couldn't send them as one command, but as separate commands. That might work in the same packet, so +++ATH\nATDTxxx but I suspect that you'd probably need a lot of \n as padding to introduce sufficient delay after the ATH.

[ssl-3]

I think I tested the combined command locally once (not with 911) and it worked. But that was 30-ish years ago and I might be misremembering. :)

Separating the commands and adding some (even small) delay would add a good amount of certainty, though. Absolutely. That's a good method. I think modems expect CR instead of LF as command termination, though, which suggests \r instead of \n.

Either way, for more haunted computer goodness: M2L3 before the dial command. This silences the dialtone and the dialing, and maximizes the loudness of the person that answers.