The question that spans the entire film is: what exactly is the black phone? Sure it’s a phone… that’s black. But there is obviously more to it than that.
When Finney first finds himself locked in the Grabber’s basement, it seems like just another random piece of junk lying around, no more helpful than the mattress. The grabber doesn’t bother on the phone, mentioning that it hasn’t worked for ages. If anything, its existence as a once-operative means of communication with the outside world is probably why it keeps it around, only to further anger its victims. Although he mentions hearing it ring once – and it was static electricity.
Finney is soon alone and startled by the shrill, old-fashioned ringing of the phone. From here, the film presents the phone as a conduit through which Finney can speak to the dead. But these are not just any souls; He can only speak to the spirits of those removed from this world by the Grabber’s filthy hands.
There’s a clear connection between the place, the black phone, and the boys who were probably killed in the exact same basement. The grabber probably has some kind of connection too; As the film unfolds, the Black Telephone is what combines killer, loot, and a haunting past.
https://www.looper.com/1019711/biggest-unanswered-questions-in-the-black-phone/ Biggest unanswered questions in the Black Phone