To send SMS/Text alerts, one has to buy an SMS Credits. You can buy SMS credits from "My Membership"-->"Buy Credits" and set rules for sending SMS from "Settings"-->"Rules" section.
To check SMS rates in your country, please select your country from the drop-down on "My Membership"-->"Buy Credits" page. The price of each SMS depends on your package and country.
Also, you as an admin can set how many hours prior to appointment SMS should be sent `Settings --> Notification settings --> Send an SMS alert to your customer (Requires the purchase of SMS Credits)`