call on vs drop in on

call on

verb
  • To visit (a person); to pay a call to. 

  • (also call upon) To have recourse to. 

  • To correct; to point out an error or untruth. 

  • (also call upon) To request or ask something of (a person); to select for a task. 

  • To select (a student in a classroom, etc.) to provide an answer. 

drop in on

How often have the words call on and drop in on occurred in a corpus of books? (source: Google Ngram Viewer )