What happens at the end of The Black Phone, and what does it all mean — here's The Black Phone explained. The Black Phone shares a lot of cast with Sinister, another horror film by director Scott Derrickson. While the two have much in common, their messages contrast with each other sharply. The Black Phone follows Finney (Mason Thames), a quiet 13-year-old boy who has a bully problem. When he's the sixth kid to be abducted by a serial killer named "The Grabber" (Ethan Hawke) and locked away in a soundproof basement, he does everything he can to safely find his way back to his little sister, Gwen (Madeleine McGraw).
https://screenrant.com/black-phone-ending-explained-meaning/