Hey Michelle!
You Totally Can!! You need to make sure the box to send SMS reminders is checked in Settings by following these steps:

• Navigate to the settings button at the bottom of the left navigation panel.
• Under Auto Client Messages & SMS which is the first purple option, make sure the box is checked.
• Then go back to settings and go into email templates - the second purple settings option, filter for appointments on the top left, and turn off the template for email reminders.

This would adjust it for all clients and not just select ones.

*If unchecked like the image below, select the check box and then click save on the bottom right.

*If checked like the image below, de-select the check box and then click save on the bottom right.