Topic: Call of Duty: Modern Warfare 2 Guide
What is the "call of duty?" Duty, is defined by Merriam Webster as "obligatory tasks, conduct, service, and functions that arise from one's position in ... a group."
Tasks and behavior that are obligatory are all ultimately voluntary. However, the "call" which precedes this statement makes shirking or ignoring these tasks socially unacceptable, and morally disdainful.