Call of duty is a legal term describing the necessity to carry out a job or duty, such as a police case, military assault/wartime actions, or other military or highly important jobs.
History
The phrase "Duty calls" has been used in media such as movies, stories, and video games, usually when someone is pulled from a conversation into their tasks. More importantly, this is used in drama, as a way of saying a sad goodbye to the family to leave for distant work. As a legal term, it is used as follows:
(Examples of charges against a person)
References
Call of duty (law) Wikipedia(Text) CC BY-SA