There is a quote from Heart of Darkness at the start. There are these kids who performed a ritual and unleashed something. Their presence at the bar caused the fight and opened a wound. The wound is a portal to another world. This is outlined in the book The Translation of Wounds II. When people look at the phone they see pictures of things the kids did and it sparks an interest or attachment in the viewer to this released entity. It comes from a tunnel. It tweaks an interest and suggests something about oneself as per the quote from Heart of Darkness. He's a shell of a man filling the void with drinking. The cockroaches are the amount of entity present. Hence at the end they're everywhere and the thing comes out of the wound portal to enter him and fill his void.
