Throughout the film, the question hangs: what is a black phone? Of course it’s a phone… it’s black. But obviously it’s more than that.
When Finny first finds himself locked in Grabber’s basement, it seems like it’s just another random piece of junk lying around, no more useful than a mattress. Grabber doesn’t worry about the phone, mentioning that it hasn’t been working in ages. If anything, its existence as a once-functional means of communication with the outside world is likely the reason he keeps it to himself, just to further piss off his victims. Although he mentions that he once heard it ringing – and it was static electricity.
Pretty soon Finny finds himself alone and winces at the sharp, old-school ringing of the phone. From here, the film presents the telephone as a conduit through which Finny can talk to the dead. But it’s not just souls; he can only speak to the spirits of those who have been removed from this world by Grabber’s filthy hands.
There is a clear connection between the place, the black phone, and the boys who were probably killed in that very basement. Grabber probably also has some connection; As the film progresses, the Black Telephone connects the killer, the prey, and the haunting past.