This is just a proof of concept. Might be useful to notify someone when a system has been booted and reached initramfs, and it is waiting for the decryption pass phrase.

• Install the required tools:

  pacman -S screen expect

Huawei MU609

• The module might be configured for packet switched (PS) only. To be able to send SMS via AT commands, the module needs to be configured for CP_PS, CS or CS_ONLY, CS = circuit switched. If you also want to set up a dataconnection, configure CS_PS. If you don't include CS, you will receive the error: +CMS ERROR: 500 when sending an SMS.

  # screen /dev/ttyUSB0 115200
  at^syscfg=2,2,3FFFFFFF,1,2

• Edit the mkinitcpio configuration file: /etc/mkinitcpio.conf:

  # Load these modules before any boot hooks are run
  MODULES="ehci_hcd ohci_pci usbserial option ohci_hcd ehci_pci"
  
  # Include these binaries
  BINARIES="/usr/bin/expect /usr/bin/stty"
  
  # Include the script and TCL dependency
  FILES="/usr/lib/tcl8.6/init.tcl /usr/local/bin/init-sms.sh"
  
  # Add custom 'sms' hook
  HOOKS="base udev autodetect modconf block keyboard sms encrypt filesystems fsck"

• /usr/lib/initcpio/hooks/sms:

  #!/usr/bin/ash
  
  run_hook() {
    /usr/local/bin/init-sms.sh 
  }

• /usr/lib/initcpio/install/sms

  #!/bin/bash
  
  build() {
    add_runscript
  }

• This is just a simple quick expect script.

  #!/usr/bin/expect
  
  # device
  set modem /dev/ttyUSB0
  
  # keep it open
  exec sh -c "sleep 3 < $modem" &
  
  # serial port parameters (probably not needed for USB)
  exec stty -F $modem 9600 raw -clocal -echo -istrip -hup
  
  # connect
  spawn -open [open $modem w+]
  send "at\r"
  expect "OK"
  
  # Unlock SIM if needed.
  send "at^cpin?\r"
  expect { 
           # ^CPIN: SIM PIN,3,10,3,10,3
           "*CPIN: SIM PIN*" {
             send_user "SIM is locked, trying PIN...\n"
             send "at^cpin=123456\r"
             expect { 
               "OK" {
               }
               "+CME ERROR: incorrect password" {
                 send_user "Wrong PIN, please configure the right PIN.\n"
               }
             }
           }
  
           # ^CPIN: READY,,10,3,10,3
           "*CPIN: READY*" {
             send_user "SIM is ready.\n"
           }
  }
  
  # Wait 5 seconds, device is connecting to network...
  sleep 5
  
  # set text mode
  send "at+cmgf=1\r"
  expect "OK"
  
  # set GSM encoding
  send "at+csmp=,,,0\r"
  expect "OK"
  
  # Configure SMS service center
  send {AT+CSCA="+31653131313"}
  send "\r"
  expect "OK"
  
  # Select TE character set
  send {at+cscs="GSM"}
  send "\r"
  expect "OK"
  
  # Send SMS to number...
  send {at+cmgs="0612345678"}
  send "\r"
  expect "> "
  
  # Text to send, end with \x1A = ctrl-z
  send "System has reached initramfs."
  send "\x1a"

• Generate new initramfs:

  mkinitcpio -p linux

• Reboot and test!