This tense, tight movie takes place entirely at a 911 emergency call centre, and almost all of the time, we see only the police officer talking on the phone. (Occasionally he interacts briefly with colleagues at the call centre.) Maintaining the tension in such a limited setting is an impressive feat for both director and star. In that sense, it reminds me of Tom Hardy's movie 'Locke', but here the stakes are a lot higher. As the main story unfolds, we get to know the people on the other end of the line and care about them. Given that we never see them, that is impressive, too. And we learn a lot about the our main character -- almost as much as he learns about himself.