1. To challenge someone to act on their threat or prove that their claim or boast is true, when one believes they are making a false claim or idle threat (i.e. bluffing). He insisted that he could run faster than me, but when I called his bluff, he suddenly said he had to go home.

2. To disprove a bluff. Whereas the first usage simply indicates a challenge, this usage indicates that the challenge resulted in the disproval. I don't know why he keeps making these outrageous claims. I've been calling his bluff for years and making him look like a fool.