If you call back somebody (or call somebody back) you telephone someone who rang you earlier, or you telephone someone for a second time.
a) Mr. Park telephoned while you were out; he wants you to call him back.
b) He forgot to book a double room, so he had to call the hotel back.
To call back is to return to a place to see somebody again.
Mrs. Ramirez is in a meeting. Can you call back this afternoon, please?