Food for thought.
You are required in most states BY LAW to have a 30 minute NON-PAID break. During this time you have full right as you ARE NOT ON THE CLOCK to do whatever you please. As long as your patients are being watched and you're available if absolutely needed, they are NOT allowed to tell you what you can and cannot do on your break. As long as your break activity is not breaking local, state, or federal laws, they cannot touch you.
Also, FWIW if this is a floor or unit specific "rule", unless this was told to you prior to accepting the position or was in writing, this is not an enforceable rule in many places. They can get you on insubordination, but more than likely this would be thrown out if taken to HR. If I were a nurse that was being isolated in a locked psych unit for 12+ hours at a time, you better be d**n skippy I'm going to have SOME form of communication with the outside world, if for nothing else than to dial 911 when your local security staff members refuse to help you with a patient that is a danger to workers, other patients, or him/herself. Believe me from experience this HAS happened, and thus they can go suck a gobstopper up a straw manually for all I care.
WITH THAT BEING SAID ... USING YOUR CELL PHONE WHILE ON THE CLOCK, ON PATIENT CARE, AND ON DUTY IN MOST PLACES WILL GET YOU AT MINIMUM A ONE STEP DISCIPLINE.