The question hanging over the entire film is what, exactly, is the black phone? Sure, it’s a phone … that’s black. But there’s obviously more to it than that.
When Finney first finds himself locked in the Grabber’s basement, it seems to be just another random piece of junk lying around, no more helpful than the mattress. The Grabber is unbothered by the phone, mentioning that it hasn’t worked in forever. If anything, its existence as a once-operational means of communication with the outside world is likely why he keeps it around, just to further antagonize his victims. Although, he does mention he once heard it ring — and it was static electricity.
Soon enough, Finney is alone and startled by the phone’s jarring, old-school ring. From here, the film presents the phone as a conduit by which Finney is able to speak to the dead. But these aren’t just any souls; he is only able to speak with the spirits of those removed from this world by the grimy hands of the Grabber.
There is a clear connection between the location, the black phone, and the boys who were likely killed in that very same basement. The Grabber likely also has some sort of connection; as the film unfolds, the Black Phone is what binds killer, prey and a haunting past.