What does this one mean? :p

Basically, it makes it so you don't get disconnected if someone tricks you into typing "+++ATH0"

It stops +++ATH0 in a IP packet (such as in a ping request, web page, etc) hanging up the modem by requiring a delay between the escape sequence (+++) and the ATH0 command.

I thought it was ATS2=255?

Either way, the memories!

S2 changes the escape character (from + to 0xFF)

S12 changes the escape sequence delay

https://en.wikipedia.org/wiki/Hayes_AT_command_set#Modem_S_r...