So this a weird one... From moment to moment "The Caller" will not be what you expect and I challenge you to guess the surprise ending.
I didn't, and I'm normally pretty good at that sort of thing. In this case, I didn't get it even with the hint that the movie is in the "sci-fi" genre. Hint, hint. Still won't help you though.
This is a story about a woman apparently trapped in an isolated cabin or small town while being visited by a strange "caller" (McDowell). Comprised largely of dialog, I feel a staginess in this production that actually helps its atmosphere quite a bit.
You'll watch like I did wondering when the "sci-fi" part will happen. Wait for it. It's worth the patience. Mostly.