SPOILER ALERT!!!!
Story of this movie revolves around two different world in the same places where they are connected with one wired home telephone. These two protagonist and antagonist communicate by making a call to one another. Surprisingly, whatever that has occurred in another world (especially death) will affects both world simultaneously.
The tone of this movie is very serious at beginning when the protagonist blames her mom for her father's death. Then, with the help of the antagonist in another world, she will save her father by turning off gas in kitchen. Everything changes beautifully in protagonist world till she know that the antagonist has killed some people which has changed again the protagonist world drastically even worse than the beginning of the movie. Again, the antagonist reveals the truth of the cause of her father's demise is due to protagonist action herself, which she consciously turn on the gas when she was little.
Obviously, we could see one of potential messages of this movie is we should face the reality no matter how bad it is to ourselves, and do not dwell on the past. Dwelling on the past by questioning 'what if's' will only make us not appreciate what do we have now and only foresee the better of us and everyone else. Perhaps, even the worst happen things for us now is the best thing for long-term as we are now stronger than ever to face even more worse reality.