Powering off Cinterion modems is done with the AT^SMSO command. Once this
command is sent to the device, ModemManager still needs to wait to get an
unsolicited ^SHUTDOWN message from the modem, which is when the shutdown
is considered complete.
Newer Cinterion devices, though, don't seem to follow this rule and just
reply OK to AT^SMSO (i.e. no ^SHUTDOWN), but instead, they will remove the
tty ports.
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