SMS Credits are required for:

Appointment Confirmations
Appointment Reminders
Messages to clients and colleagues
Direct Messages and Smart Forms
Identity verification, for online booking (optional)

To purchase SMS Credits:

You will be taken to the Buy SMS Credits page
Select your preferred bundle
Follow the payment instructions on screen
If successful, you will be taken back to the Account Details tab in WriteUpp where you will see your SMS Credits counter is updated with your current total: