I completely understand why an institution would create a 'smartphone use' policy. But at the same time, how little do you trust the people working for you? How broken is your culture of professionalism that people now have to announce why they are using their smartphone? Do you also make them ask permission to go to the bathroom?
First and foremost, attendings and senior residents must set the example. Then, these same supervisors need to keep a casual eye out for interns and med students misusing devices. If that were to happen (which is much less likely if the attendings and senior residents create a culture of professionalism), then pull the junior member of the team aside and address the issue.
I don't think this requires a separate policy and I don't think the creation of such a policy is something to celebrate.