I had very little expectations from this film. It didn't look like it had much of a budget and no actors that I'd heard of (okay, so I sort of recognised the main woman from Twilight, but that was about it). However, in this case, less is slightly more (or at least slightly more than the average dross that infests modern horror films).
I wouldn't call The Caller a horror, more of a supernatural thriller. It's about a single woman (aren't they all?) who gets some mysterious phone calls. Nothing too out of the ordinary, but it presents a pretty creepy premise which finds her trapped in a spiralling cycle of torture and paranoia from a very nasty supernatural adversary.
The Caller is a `slow burner.' This is why it's probably not for everyone. Nothing too out of the ordinary happens in the first half and I can see a lot of people giving up before it really gets going. However, the second act cranks it up a gear and the torment really begins. There are no major scares or gore to speak of; it's more a case of inescapable mental torture.
If you can put up with your films a little slower than normal and without any action, effects or gore then give it a go - all performances better than your average horror/supernatural flick and a slightly different premise which really works.