It's a python script.
1. Open this script, set your phone number in variable (my_number)
2. Run this script once, you'll get your imsi code in message
3. Open script again set the IMSI code in (my_imsi)
4. Change the variable (my_number), set the number where to send the message if phone is stollen.
5. Edit the crontab and set desired period to run this script.

Also you can read a comments in the script configure it freely and also can change it to.
If you have any ideas tell me or join to my project.

You'll agree with me: The thief in many cases has no tech knowledge. So after stealing your phone he will change the SIM card, so when when phone is on this script will check the IMSI that in script with IMSI of current SIM card. If IMSI in script does not equal with IMSI of current SIM card it will send a message to number which you set in the script (my_number - variable) in given periods (crontab)
In my country (Azerbaijan) it's easy to find the stollen phone if you know the phone number and / or IMSI code. Thiefs in Azerbaijan has no tech abilities. If he would be a "techman" he wouldn't steal my phone.