How to use a console SIP client on your Armbian device to send text messages to a SIP client on your phone or PC.

If you use a SIP Voip client on your phone or computer, you can use the sip protocol and scripting to monitor your Armbian board. 

I'm using Ubuntu Xenial, but other versions should work as well.

I have the Linphone client on my phone and I use the free Linphone SIP service.

Linphone.org hosts a free SIP service that allows users to make audio or video calls using SIP addresses via the domain sip.linphone.org. 
You can also send text messages.

SIP requires an internet data connection, and my phone is connected via WIFI most of the time except when I'm commuting, so I set up my monitoring chron jobs on the Armbian device to run at times when I'm connected.

Set up a SIP account for yourself if you don't yet have one, and set up an account for your device.
You can set up free accounts at https://www.linphone.org/freesip/home
The example accounts used in this tutorial are: sip:armdevice@linphone.org and sip:youraccount@linphone.org
Replace these with your own accounts.
 

I suggest that you get your Voip service running and tested on your phone and another device or PC before you try and set up SIP scripting on your Armbian device.

Install the Linphone console client on your Armbian device (Orange Pi Zero Plus in my case).

sudo apt-get install linphone-nogtk

When it has installed start the client

linphonec

You may get a bunch of warnings about unconfigured sound cards and etc.  Unless you want to use voice or video, you can ignore these.
........ eventually you will get a linphonec prompt
Warning: video is disabled in linphonec, use -V or -C or -D to enable.
linphonec>

If you type help, you will get a list of commands:
linphonec> help
Commands are:
---------------------------
      help      Print commands help.
      call      Call a SIP uri or number
     calls      Show all the current calls with their id and status.
      chat      Chat with a SIP uri
 terminate      Terminate a call
    answer      Answer a call
     pause      pause a call
    resume      resume a call
  transfer      Transfer a call to a specified destination.
conference      Create and manage an audio conference.
      mute      Mute microphone and suspend voice transmission.
    camera      Send camera output for current call.
    unmute      Unmute microphone and resume voice transmission.
playbackga      Adjust playback gain.
  duration      Print duration in seconds of the last call.
autoanswer      Show/set auto-answer mode
     proxy      Manage proxies
 soundcard      Manage soundcards
    webcam      Manage webcams
      ipv6      Use IPV6
       nat      Set nat address
      stun      Set stun server address
  firewall      Set firewall policy
 call-logs      Calls history
    friend      Manage friends
      play      play a wav file
    record      record to a wav file
      quit      Exit linphonec
---------------------------

Now you need to configure your SIP proxy.

"help proxy" gives you the relevant commands

linphonec> help proxy
'proxy list' : list all proxy setups.
'proxy add' : add a new proxy setup.
'proxy remove <index>' : remove proxy setup with number index.
'proxy use <index>' : use proxy with number index as default proxy.
'proxy unuse' : don't use a default proxy.
'proxy show <index>' : show configuration and status of the proxy numbered by index.
'proxy show default' : show configuration and status of the default proxy.

We must use "proxy add". It will start an interactive session. Note the format of the proxy sip address enclosed with < and >

linphonec> proxy add
Adding new proxy setup. Hit ^D to abort.
Enter proxy sip address: <sip:sip.linphone.org;transport=tls>
Your identity for this proxy: sip:armdevice@sip.linphone.org
Do you want to register on this proxy (yes/no): yes
Specify register expiration time in seconds (default is 600):
Expiration: 0 seconds
Specify route if needed:
No route specified.
--------------------------------------------
sip address: <sip:sip.linphone.org;transport=tls>
route:
identity: sip:armdevice@sip.linphone.org
register: yes
expires: 0
registered: no
--------------------------------------------
Accept the above proxy configuration (yes/no) ?: yes
Proxy added.
linphonec>
Password for armdevice on "sip.linphone.org": Y0uR-pa$$word

linphonec> Unregistration on <sip:sip.linphone.org;transport=tls> done.

That's it you are now configured. (not sure what "unregistration" means, perhaps it's a typo and should say registration)

Send yourself a test text and quit out of linphonec.

Once again "help" is useful:

linphonec>
linphonec> help chat
chat <sip-url> "message"' : send a chat message "message" to the specified destination.

linphonec> chat <sip:youraccount@sip.linphone.org> "Testing sip messaging"
linphonec> terminate
No active calls
linphonec> quit
Terminating...

How can you easily send messages from a script?

Create a wrapper script using expect (uses tls not bash)

If you don't have expect installed then

sudo apt-get install expect

Create a wrapper expect script called "send-sip.sh"

#!/usr/bin/expect -f
# send-sip.sh "this is a message"

set message [lindex $argv 0];

spawn linphonec
expect "<sip:sip.linphone.org;transport=tls> done."
send "chat <sip:youraccount@sip.linphone.org> \"$message\"\r"
expect "linphonec>"
sleep 10
send "terminate\r"
expect "linphonec>"
send "quit\r"
expect "Terminating..."

exit

Set execute permission on the file. I placed mine in /usr/local/bin

chmod  +x /path/to/file # e.g. chmod +x /usr/local/bin/send-sip.sh

Now you can execute your wrapper from a script like this and it will send the message to your sip address.  Change the path to the location of your wrapper script.

/usr/local/bin/send-sip.sh "this is a sip message"