The mysterious caller

She looked around but didn't see anyone suspicious. With a smile remaining on her face, she pretended to be calm and entered the site. The organizer took her to her seat and told her to take a rest. Each star was assigned a dressing room behind. Isabelle went to the dressing room for a makeup touch-up, and Renee was carrying some bags in her hands, inside which were clothes that Isabelle would need later. While Isabelle was having her makeup touched up, her cellphone rang. She looked at the strange caller ID and hesitated. Eventually, she picked up the phone. But she did not hear anything after pressing the "answer" button. "Is this some kind of prank?" she wondered. "Isabelle, your makeup is done." "Um. You may all leave. I need to rehearse by myself." "Okay. We'll be waiting outside for you." After they went out, Isabelle was left alone in the room. While Isabelle was thinking about the speech she was about to give, her cellphone rang again. The rapid bell resounded in the large room