how about creating a separate employee area (with changing area, lockers, restroom, etc) that is separate from the store. an entrance to the store with metal detector & security would settle this nicely. employees would be allowed in with their uniform (t-shirt & jeans) and nothing else, perhaps an i.d.
i realize i'm in the minority here, but imo, people don't need their personal phones on them while working. if they need phones, the company should provide them.