Mobile phones are banned during exams. We have to put up notices to that effect. If pupils arrive at an exam with a phone, it has to be handed into the invigilator.
Invigilators can have mobiles, but they have to be switched off. They are only to be used to summon outside help, so can be switched on at the time. In my school, we only ever have one invigilator, so it is essential to have a mobile available.
As for general mobile policies, I think it is a bit hypocritical to ban mobiles completely. As parents, we are all supportive of our DCs having mobiles. It really takes a lot of pressure off us to know that they can contact us if ever they get into difficulty. Teachers generally have mobiles and it makes a big difference having one - it removes one of the restrictions of being a teacher. I imagine that in certain schools it is particularly handy in summoning help in emergencies.
For our pupils, the kosher use of mobiles is to pass messages back and forth about pickup times (eg parent stuck in traffic), and therefore only need to be switched on at the end of the last lesson. If the pupil is sick during the day, they call from the school office.
My own kids never have their mobiles switched on which can be infuriating, but I know that if they need to contact me they can (although last week would have been bad because I had a dead battery for the week )