9.3. Use or configure a dumb modem
Most modems use the Hayes AT command set.
The modem's attention is gained by sending +++
surrounded by some idle time. Commands are then sent prefixed by
AT.
Unfortunately, if the modem sees +++
during a call it may revert to command mode. The modem can then be
configured by the caller. For example, the modem could be set to
permit incoming calls only from the number "0", this
would deny the system administrators access to the modem.
The attention command can be removed using AT
S2=255. Of course once that is done no more
AT commands can be given to the modem, so any
other configuration of the modem needs to be done prior to that
command.
Unfortunately, when power to the modem is applied the modem
starts in command mode. So a carefully chosen console message
could disable the modem.
The best solution is to select a modem that has a
"dumb" or "select profile"
DIP switch or jumper. These switches disable
command mode and load the modem's saved configuration when they
start.