That box doesn't exactly look safe to me. Not just vulnerable to theft, but also to mistakingly taking someone else's phone. And for phones with NFC, it's a great way to have your mobile wallet emptied.
On sites with classified areas, they'll often have little individual cell phone lockers right outside the turnstile to the limited area. Drop in your phone, lock it, take the key. That would be a much better way to control the phones.
