Unenforceable, especially at scale. (Do you want to be responsible for collecting and returning 1k+ cell phones per exam?). The phone doesn't have to be visible at all for the system to work, just near enough. And never mind the diabetics who use their phone to check blood glucose readings and control their insulin pump...
At my place of academentia, we're not allowed to have students leave their bags at the front of room anymore - they're allowed to keep them under their seat. (If you're wondering why, imagine a single phone going off during an exam in a backpack in a pile of 1k+ backpacks...)
And before you suggest it no, blocking cell phone signals for all exam rooms is not practical (nor legal where I am).