To call claim is to express your beliefthat an exclamation is untrue. The act of calling claim is a challenge to the person to go through with the action, often one that is very ambitious or daring. The expression is often used to make a person publicly backdown or bring light to the ambitious statement made.
Scott: Dude, am i fat?
Scott: I'm going to come back from summer so jacked!
Friend A: I call claim.
Scott: *whiny voice* No, seriously...