Do you frown upon cell phones in the classroom?  I do.  Cell phones are a disruption, even if they are on vibrate.  The student is distracted from the vibration, and others will be distracted if the student leaves the room even quietly to answer it, it is not a big disruption, but a disruption.  Students made it just fine in school before cell phones became so common, and they can make it just as well without having cell phones on in the classroom.  Some jobs have a no cell phone policy.  You either abide or else . . .