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.
Here then, the phrase "call of duty" makes a set of tasks very obligatory. Those who ignore it are free to do so, but at the cost of their own conscience and contribution to the good of society.
