Behind the Mask (2018)

Teen girl Melissa Moore begins to receive anonymous phone calls from a stranger, who does nothing but breathe into the phone. At first, she is creeped out by the calls but, as they keep happening, she realizes that it's probably just her boyfriend playing a joke on her. So, she starts to talk back to the "stranger," playing along, and threatening to find him and expose him for being a pervert ... referencing the fact that she knows who he is! But, no matter how much she eggs him on, he never speaks or reacts. She begins to anticipate his calls and fantasize about their roleplay ... even masturbating about it.