I watched this at the festival where after the projection we had a QA session with the director.
The Film itself is technically impressive, all framed within 2 rooms where all you'll see is the main character at the phone and very little more. This is not a limitation but instead the strength of the Movie itself; it's all about guided imagination. The dialogues push your figurative skills to build this outdoor world that is intentionally not shown to you.
There has been a lot of background work/preparation for this Film and it's all visible in the final product.
The director did a great job alternating long moments of high tension with some short funny ones to engage the audience.
I do consider this Movie very unique and original but you might find some similarities with Locke and perhaps also Phone Boot where a series of phone calls is the actual stage.
Unmissable title for 2018
Bravo!