call box vs letter box

call box

noun
  • A box containing a telephone for use in emergency, usually mounted on a post in an area where normal access to telephones is inconvenient (such as on an interstate highway or a very long highway bridge) 

  • Synonym of telephone box (enclosure) 

  • The wall-mounted box allowing a person to speak into an intercom 

letter box

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