Ryan Reynolds does a great job in this movie considering he has very little to work with. And I'm not referring to the "claustrophobic" surroundings that are meant to create the film's tension. I'm talking about the lack of story. The concept of the movie is meant to carry it the whole way, so they don't bother to introduce any element for us to actually care about.
Reynolds wakes up to find himself locked in a coffin and spends the first 10 minutes of the movies grunting and screaming. He then proceeds to call a mindless parade of remarkably useless morons on the world's greatest cell phone (he's calling from a coffin in the ground, remember). Just when you're ready to give up he gets a call from a terrorist demanding a ransom. The story maintains this level of absurdity until the end, where it peaks.
I don't believe any of the characters. Everyone but Reynolds is unforgivably two dimensional, so you can't accept that a world even exists outside that box. The trouble is, if I don't believe the world outside the box then I can't invest anything in what's happening inside it.
I also think the cell phone generation is more enamored of toys than they should be. I understand that this was the device they needed to give him a link to the world, but the tension built around it felt more like the aggravation one feels when trying to connect with tech support than a real crisis situation.
The whole movie's designed to prey on the viewer's personal sensibilities and so we never really develop any empathy for the character. The suspense is contrived and predictable, and sometimes they just do something dumb to break their own boredom. At one point an asp gets into the coffin and Reynolds has to MacGyver it with the contents of his flask and his trusty Zippo (what is he drinking, pure kerosene?)
The tone is also wrong. They're more interested in modeling this movie after torture-victim tragedies like Saw than legitimate character suspense thrillers. They should have watched "Phone Booth" instead. That movie did the same concept, but it actually worked. Or "Closet Land", which was much truer than this idea and disturbingly effective. This movie just felt like overly ambitious filmmakers experimenting with an idea they didn't fully understand.