1. You do nothing and the appointment stays in your calendar as an "unconfirmed" appointment.
2. The customer can phone you directly and ask you to confirm the appointment manually.
3. You can call the customer yourself to check that they happy with the
appointment and then click on "confirm" in your administration screen.
We will take an action out of this question to improve the customer experience so that they are prompted to call you if they don't get the text message confirmation.