How are you sending SMS?
To send SMS you'll need to go through gateway, and that's dependent on the carrier.
Typically speaking, you'd need some API that handles one-way/two-way messaging. As well, depending on country, there are many regulation surrounding sending of SMS via automated system.
I would advise against going down this path, without getting advise from your company's legal team. Even then, I'd recommend using API service for sending SMS rather than in-house coding.
NOTE: Specific countries will have different regulation. North America, for instance, requires that messages must be sent from a provisioned, text-enabled number (i.e. must always be two-way), and that it has code to stop sending messages.