If you are a film student you know that the "reversal" usually comes in the final act, and there is a reason for that. By that point, the viewer is fully engaged, the endorphins are pumping, the action is flowing, and there is a need to take things up a notch.
What is happening here -- and let's give credit where credit is due -- is that the reversal takes place off the top, two pranksters get the tables turned, and it all happens in the first 10 minutes.
That is clever and it is a great start. But it creates problems. If you bring the reversal in too early, you run the risk of becoming dependent on it to drive the film, instead of relying on the basics. Such as dialog, characterizations, empathy, viewer connection.
Which is what happens here. After a great start there are major chunks that just lag, and the reason is that -- to be blunt -- by the halfway point, if you are a typical viewer, you don't really care if the 'slasher' gets the kids or the kids get the slasher. NO EMPATHY.
Many of the subsequent "slasher tropes" that close the third act are just that -- borrowed tricks from dozens of films you have already seen.
Overall the film is technically perfect and at some point in the not-too-distant future, those involved in it will look back at DON'T HANG UP as a great start for their Highlight Reel, but not entirely representative of their best work.
