If you're on the phone with someone, and someone else calls, they enter a call waiting face off. The loser obviously being the one you tell you'll call back.
A: "Hey, hold on. I got another call."
B: "'k. (Goddamnit, I'm in a call waiting face off)"
A: "Sorry about that, I was on the other line."
C: "It's ok. (Fuck... call waiting face off.)"
A: "Hold on a sec, 'k?"
C: "Sure. (SUCK IT, I WIN!)"
A: "Hey man, I gotta call you back."
B: "Alright, see ya. (Fuck, I lost.)"