Is it possible to notify users of a modification of the timetable by SMS?

Yes.

In the module Daily management and absences, go to the timetable of the concerned resource, select the course on the grid, then click on the button at the bottom left of the course form. In the menu that appears, select the option Send by SMS; in the window Editing and sending of SMS, select an SMS template (created beforehand) from the list on the left, or create a new template yourself.
Once the template has been selected, on the right-hand side, verify (if the template has already been created) or write the content of the SMS.