call to arms vs sign on

call to arms

noun
  • An appeal to fight or to engage in conflict. 

  • An appeal to undertake a particular course of action or to support a specific cause; a call to action. 

sign on

verb
  • To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours. 

  • To commit oneself, as to a project, a goal, an organization, a cause. 

  • To join something, after signing. 

  • To apply to receive unemployment benefits. 

  • To log on; to start using a computer, radio, etc., or to start talking. 

noun
  • The time of day when a radio or television station begins broadcasting, usually after being off the air for several hours. 

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