Go to leaderboards in your menu.
Go to feat scores.
Using
/
, pick any feat you already have a score in.
Press
to choose a player to call out. You'll get a list of friends (regardless of whether or not they have the game) and other people who are playing the game.
Choose a player and press
to call him out. Trophy unlocked!
Note that if you have a friend who has the game, if you send him a call-out and he can beat it then he can get the following trophy:
So have someone call you out as well, either by asking a friend or calling out a random player and hope they return the favor.Update: It seems you can get that other trophy if you just beat your own score on which you called someone out, you don't have to wait for them to call you out.