headphone vs phone

headphone

noun
  • A cushioned speaker placed on, over, or inside the ear, normally used in left/right pairs. 

phone

noun
  • A device for transmitting conversations and other sounds in real time across distances, now often a small portable unit also capable of running software etc. 

  • A speech segment that possesses distinct physical or perceptual properties, considered as a physical event without regard to its place in the phonology of a language. 

verb
  • To call (someone) using a telephone. 

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