Significant Other (2007)

Louis has liked Christine since he first met her, but she's never thought that way about him. When he finds out she's dating Curt, yet another jerk who will just use her and throw her away, Louis takes matters into his own hands by calling her from a blocked number and pretending to be a stalker. His hope is that she'll come to him for comfort, and then realize she should be with him. It all seems to be working; she's opening up more than ever and her boyfriend is insanely jealous. But in a strange turn, Christine reveals that she enjoys the attention of the imaginary stalker. Meanwhile, Curt nobly offers to catch the stalker, and discovers that it's actually Louis. He thinks he's won until Louis reveals his ace: photographic evidence that Curt is cheating on Christine. The two form a shaky truce, but Curt opportunistically turns Louis in to the police for stalking not Christine, but the girl Curt was hooking up with. And he has the photos to prove it. Louis is expelled from school, and Christine comes to say goodbye as he's packing up his room. But Louis brushes her off, realizing that his efforts were misdirected in the first place.