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Phrase(s): Who’s on the phone? [and] Who’s on the line?
Who is on the telephone line now?; Who just called on the telephone? (The caller may still be waiting.) • Bill was on the telephone, and Mary walked by. "Who’s on the phone?" asked Mary, hoping the call was for her. • Tom asked, "Who’s on the line?" Mary covered the receiver and said, "None of your business!"
McGrawhill's American Idioms And Phrasal Verbs