How do you have your phone with you? Stash it in the bathroom? At my university the invigilators make you show what's in your pockets before and after going to the bathroom (which has itself been checked, and which has a log of people and times), and you can't take anything out (e.g. formula sheet to secretly copy somehow-bathroom-stashed notes onto) or bring anything in. Phones are in bags up the front of the room, inaccessible. The kinds of things you're talking about very rarely happens, and consequences include fines and academic penalties.