If you want to put your custom instructions on how your customers can confirm/reschedule/cancel, you can include that in the message template itself.
Note: Even if the automatic instructions are disabled, replying C will always confirm, replying R will always reschedule, and replying with X will always cancel an appointment.
To do this, disable the settings on the Settings > Reminders page.
scroll down to the "Confirm, Reschedule, and Cancellations" section.
Uncheck the boxes to add the Confirm, Reschedule, and Cancellation instructions as seen below.
Then scroll down and click "Save".
Then you can add your own instructions if you want.
Go the the "Message Templates" section on the Settings > Reminders page.
From the drop-down menu, select the template you would like to edit.
Add your own instructions on how you want your customers to confirm/reschedule/cancel their appointment.
Example:
Reply C to confirm. If you need to make changes to this appointment schedule, please call our office at 555-555-5555.
You can click "Preview Reminder" under the message box to see a sample message.
Click Save.