Another reason for banning mobile phones in class is their potential use for cheating. In Russia, where I live, the introduction of the single state exam for school leavers two years ago was marred by widespread reports of cheating; students took phones into the exam halls, texted the questions to friends (and sometimes to teachers who'd agreed to be their accomplices) and received return messages with the correct answers. (This applied to multiple choice questions, which constitute the usual form of exam questions in Russia; obviously phone message services would be of limited use where exam candidates were required to write essays).
I wholeheartedly agree with members who advocate banning mobile phones altogether from classrooms, but that's easier said then done, and the students' age and status is relevant. Adult students, particularly working adults who pay for their part time education, resent being given orders, so the teacher has to be very tactful; all the same, there will always be someone who flatly refuses to accept a moratorium on calls during class time.