First off, that handbook is in its third year, and if that rule really meant that students couldn't use their cell phones during the entire meet, whether they are actively competing or sitting in the stands picking their nose, you would have heard about it by now. How many high schools are in Missouri, and you think officials are policing the stands demanding kids turn off their phones? I live in Missouri, and I assure you that is not happening.
Apply the same definition of competition used throughout that handbook to the cell phone rule, and it's pretty clear MSHSAA doesn't want you to use your phone while actually competing in an event. For instance, anyone who reports to the long jump area to check-in after the judge has started the competition will not be allowed to participate in the event. That doesn't mean an athlete has to check-in before the meet starts, it means he/she has to check-in prior to the judge starting the long jump competition - you know, when athletes are actually jumping or waiting their turn to jump. Similarly, in the instance of a tie in the High Jump or Pole Vault, the "Tie breaking competition should be held at the conclusion of the field event." Again, while actually pole vaulting or waiting to vault, not while sitting in your team tent scrolling through the letsrun messageboard.
Whether you're actively competing or waiting your turn, MSHSAA doesn't want you to use your phone, which actually makes a lot of sense. How many football or basketball players sitting on the bench do you see using their cell phones during a game?