Compaq Armada 1700 Reference Manual page 82

If you enter a command that normally includes a numeric
parameter (such as ATHn) without the numeric parameter, the
modem interprets the missing parameter as zero. For example, the
Hn (hang-up) command can have a numeric parameter of zero
(H0) or of one (H1); however, if the parameter is missing,
ATH<cr> is the equivalent of ATH0<cr>. This rule applies to
command parameters only, not to dial strings.
The escape code sequence (a 3-character code surrounded by
escape guard times) forces the modem from the online mode to the
command mode. You must enter the escape character three times
consecutively, and the delay between entering each escape
character must not exceed the escape guard time. After a short
delay, the modem responds with the OK result code to let you
know that it has successfully "escaped" to the command mode.
Use the ATO command to return to the online mode.
The escape guard time is defined as the time delay required
between the last character transmitted and the first character of the
escape code. The default escape character (set in register S2) is +,
the default escape guard time (set in register S12) is one second,
and the default escape code sequence is +++.
You can repeat the last command line issued by entering the A/
command. This action also repeats the AT prefix and the <cr>;
therefore, you are not required to enter the AT prefix or a <cr>
when using this command.
Each string command remains in the command buffer until an AT
is entered or power to the modem is turned off. Both actions clear
the buffer and make the A/ command invalid, since there is no
command for the modem to repeat.