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.
(..) - Yeah, like the beverage. Yeah, she's here. What does Mr Powell do? He's a lawyer and I'm his law clerk. A fairly recent appointment, but enjoying it, all the varied work. - A lawyer? - Isn't it obvious? Lady Croft, my pleasure. - Manfred Powell, QC. (..)2001 Lara Croft: Tomb Raider