have you seen the Reengagement activity?
https://moodle.org/plugins/mod_reengagement
We've just added code to the version in github that now uses the Moodle notification API instead of just sending e-mails around - you could install a messaging provider that sends sms messages:
https://moodle.org/plugins/?q=type:message
then configure the student notifications to send SMS messages for certain messages like those from the reengagment activity.
let us know how you go!