I actually see that pretty often. When the movie requires it, people forget that they have cell phones. It's the same way the horror movie universe apparently doesn't have horror movies, or people would know not to split up or have sex while camping.
...I actually wonder why they don't just pretend cell phones don't exist.
Because it feels forced. Ghosts and zombies are the premise of the stories, a thought experiment around which the plot unfolds. Pretending cellphones just don't exist feels like a contrived fix to the writer's inability to write a good plot. It's equivalent to a deus ex machina. Unless, of course, you can write a plot that hinges on the lack of cellphones - that could be interesting.