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.
(..) But I'd rather end up wishing I hadn't than end up wishing I had. Wouldn't you? I don't know. Some man came without an appointment, Your Excellency. Waiting outside. Levin! Where did you disappear? I'm very anxious to see you. I need your advice. Well, come into my room, then. (..)2012 Anna Karenina