There is a simpler explanation: I once worked an (industrial+officies) site of high-tech company, and they had their own private Cell tower, with phones that on

There is a simpler explanation: I once worked an (industrial+officies) site of high-tech company, and they had their own private Cell tower, with phones that only worked on the site; maybe they forgot to update the time. IIRC, small base stations have been deployed at hacker events(CCC) with similar internal-only features.