The term 'CALLER IN A MASK' is a job description for an UMP, which stands for umpire. An umpire is a person who officiates a sporting event, such as a baseball game, and makes sure that the rules are followed. They also make decisions on plays and calls, and can issue penalties if necessary. The term 'CALLER IN A MASK' refers to the fact that umpires often wear masks to protect themselves from being hit by balls or other objects during the game.