In terms of passive signal blocking, I wonder if the chickenwire mesh that holds exterior stucco in place could serve as an effective Faraday cage? Or aluminum siding? I'm sure the metal roof and walls of a modular storage building would.

Most businesses use metal studs to support interior walls, though I doubt their ~2 foot spacing would block wifi or cellular RF.

In terms of active cellular jamming, that should be pretty easy to detect -- by standing in an outside doorway where you'd be exposed to inside AND outside signals, and then checking if your cellular bars fade out. That should indicate that you've been exposed to an internal jamming signal, since the external signal should still remain.

It's hard to imagine that the advantage to a corporation of isolating the customer's phone would be worth the risk of FCC prosecution.