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At a tense moment when they need to contact authorities, the heroes of the film discover that they have no cell phone reception.
In many movies, you'll notice that the characters never have to worry about things like "Is this call/text covered under my plan?" or "Do I have a clear signal?" because the call will always go through when the plot calls for two characters to talk to each other. In this age where almost everyone has a cell phone (even poor people can afford to buy a temporary one), no one really questions the fact that when two people have to talk to each other, then they will be connected without much fuss.
But just as people usually expect their call to go through when they call someone on their cell, they also know very well the problem that arises when that connection is broken, so if you need to build some tension between two characters in a modern-day movie, then have one or both of them suddenly lose their cell phone reception. Cut off from their friend and others who could help, they'll be forced to rely on what tools and information they currently have with them to progress any further. Can also be used to raise a misunderstanding of the "why didn't you call me?" variety to be played either for laughs or for actual drama.