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.
(..) You know I'm not serious, right? Of course. You know how lucky we are to have found each other? I've got a very important appointment. - Is it okay if Angela sleeps over tonight? - She's always welcome. I thought you two had a fight. I haven't seen her around here in a while. What? (..)1999 American Beauty