Dude2: "Dude, it's 2:30 in the morning."
Dude1: "Yeah, I just got home from the bar. You, uh, you want to come over and play some Modern Warfare 2?"
Dude2: "Dude, I can't be your Duty Call tonight, I have work in the morning."
Dude1: "C'mon, I need you... to help clear out the favela on veteran in spec ops mode."
Dude2: "I'm there, my job sucks anyway."
When your you're chilling on a sofa and suddenly your baby starts crying and you gotta check it out, then you might say: Oops! Duty calls!!
"Oh no thank you Jeff. I heard Janet from down the street lost her husband in Iraq, so i have duty call."