We are calling to 'confirm your appointment' with Dr. Jones. = We are calling to make sure that you will 'keep your appointment' with Dr. Jones.
The museum is open to visitors 'by appointment' only. [=you have to make an appointment to visit the museum]
The court ordered the 'appointment' of an attorney to represent the child.
(..) - Simin, where're you going? - I'll be right back. Just to the corner. I'll be right back. I'll come back. I have an appointment. Four. Why four? Because it has faded. She must have used it the most. Four it is. (..)2011 A Separation