Hi there @CristianaSP, welcome aboard! Perhaps one of our latest Advent Calendar apps might help you out here - it explains how to integrate the external service Nexmo to deal with SMS to users. And here’s another app with SMS to look at for inspiration.
I’m not sure about the current pricing of Nexmo for what you would like to do, but they do offer a free trial. So you could give this a go and see if it matches what you want. Hope this helps!