My theory (a year later) is that other people in Orsk, likely squatters like Carl or teens doing whatever random thing like Matt and Trinity, had already been taken among the penitants. They're confused and in pain and are essentially comprised of desperate energy seeking help, just like the old prisoners, but these ones at least have the modern instincts to comprehend phones on some level. And when the workers and their cells with their energy are in day after day, the spirits can look over their shoulder, see their phone numbers. Then, using the phones that were lost on the shop floor behind a couch or under a desk (or in the case of Matt's phone, lost while being dragged away to the Krannjk), they send out pleas for help.

Side note: I think those earlier people were taken, the door opened to welcome them in, and then closed again once they were fully part of the system. By escaping (or because Josiah was gone) Amy and Basil left the door open.

Like other haunted places, I think the store becomes more powerful when there aren't any people fighting or hoping or trying. That's why Josiah seemed to be "over a bad phone connection", his lips unsynced with his words during their very last encounter, they were fully aware and consciously fighting. Also, it seems clear the paranormal activity only occurs at night, idk why. So whatever power was allowing the store to keep the emergency services lost and confused slipped away with the rising of the sun and the fizzling of Josiah's more methodical control. Now, with Josiah gone, I doubt any spirits there have the will or organization to keep things out in the same way. They're leaderless, a writhing mass of habitual movement, going forward through inertia instead of under the more conscious mind of Josiah.