To "call someone out" means to challenge them to physical combat. A duel of some kind. In times past this meant an encounter with swords or dueling pistols. More recently, it meant a fast-draw shoot-out in the middle of the street.

You're "calling people out" Martin Gibson? Sounds real big as you hide there anonymously at your computer.

But you don't have the guts to "call" anybody out for real.

All mouth. No backbone.