DISCLAIMER: I did not see this movie in its original language(Danish), but in a dubbed version(Italian). I may have missed some nuances.
My main worry going into this movie was that it would be far too introspective. Thankfully, a short runtime(about 90 minutes) and great acting/directing addressed that problem in a brilliant way.
The story starts slow and then gets more interesting as the movie goes on. Without spoiling anything, I'll say that while there is only one character and one location on screen, there are many more that you don't see but instead only hear. That keeps things fresh as the protagonist moves between different situations that the viewer don't see, but can imagine.
Great performance by the actor, it's not easy to act in a believable way when you are just talking on the phone without no one in front of you. Well done.Not totally satisfied with the voice acting of the people on the phone, but that is probably limited to the italian version I saw.
Good work on the cinematography. While the location was pretty basic, every shot was well framed(I especially liked those in the dark) and they helped convey the emotion of the police officer even when he was not talking.
The script was written with very concise dialogue( as one would expect to find in a 112/911 situation).
Overall, a very good thriller that will keep you on the edge of your seat without showing you basically anything. Nice story with interesting psychological implications that are not excessively explored. Almost a must-see.