Compaq Armada 1700 Reference Manual page 81

You must follow certain rules when entering or using the AT
command set. Read the topics in this section to learn more about
these guidelines:
You should precede all commands except A/ and +++ with the
attention code AT.
You can enter commands in uppercase or lowercase letters or a
combination of both. However, always type the AT prefix in the
same case (not At or aT).
You can enter a maximum of 60 characters for the internal modem
following the AT. The modem doesn't count the AT prefix,
carriage return (<cr>) character, or spaces. It counts, but doesn't
act on, punctuation, such as hyphens and parentheses.
You can enter telephone numbers with or without punctuation; for
example:
(123) 456-7890 or 1234567890
However, punctuation in the telephone number takes up space in
the command buffer.
If a command sequence exceeds the 60-character maximum, the
modem does not execute any part of the command line and returns
an error message after the <cr> instead. If this occurs, reentering
the command within the 60-character limit allows the command
line to be executed.
If you make an error while typing a command, simply backspace
over the mistake, then retype the line. You cannot backspace over
the AT prefix because it is interpreted immediately after it is
typed.
After you enter an AT command, execute it with the Enter or <cr>
character (selected in register S3).