The ClearPass Guest SMS service is only available for testing purposes. It allows in proof-of-concepts or tests to send some SMS text messages to demonstrate the workflow.
Aruba does not offer subscriptions or credit upgrades and for actual deployment you should get your own SMS service. Some suggestions are in the drop-down when you add a SMS Service Gateway, many others can be added through custom HTTP or SMTP.
Basically, you can get everything tested before you purchase a third-party subscription.