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. In general, each SMS costs 5 cents.
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)`