Stu's situation; he's trapped in a phone booth, held hostage by a sniper who's aiming a high-powered rifle at his head while making conversation.
The shooter escapes. To terrorize god knows how many more victims like Stu.
The Caller in general. He's been watching Stu for a long time, knows all the details of his life and prepared for the events of the film well in advance. It's made painfully clear that he could have killed Stu at any time if he wanted and his final scene makes clear he'll be keeping an eye on him. Imagine spending the rest of your life, knowing someone like that could be watching everything you do and could kill you at any time if he decides he doesn't like your behavior and considers simply being an insensitive jerk worthy of a death sentence.
