In my opinion, a judge would not be out of line to forbid cell phones in the courtroom at all. Nor would he/she/it be out of line to find a person in contempt if the person were warned to turn off the ringer and intentionally failed to do so.

To hold a person in contempt because the person made an honest try to turn off the ringer and did it wrong, would, IMO, be an abuse of judicial power. But a certain portion of the population are utter assholes, and the selection process is not perfect, so some of them make it to the bench.

Peter