letters vs voicemail

letters

noun
  • Literature (school subject). 

  • The liberal arts, humanities, learning (broad accumulated cultural knowledge). 

voicemail

noun
  • A computerized interactive system for storing, processing and reproducing verbal messages left through a conventional telephone network. 

  • An individual message on such a system. 

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