This installment is the strongest of the "Whispering Corridors" series. The rule system regarding the communication between the ghosts and humans is interesting, and is utilized well when everything comes together near the end of the film. There is limited (yet successful) use of special effects and the night time atmosphere is very good, using dark red lighting on numerous occasions that rivals the beauty of Dario Argento's "Suspiria." Everything about this movie screams quality, from the interesting storyline to the excellent acting to the effective, stylish horror elements. The scene that plays during the closing credits is something really special. This is a beautiful film with fantastic scriptwriting.
The sense of friendship is much stronger in "Voice" than in any of its predecessors. This is due mostly to the fact that the movie is seen through the eyes of the ghost instead of the living schoolgirl, which automatically humanizes the spirit instead of portraying it as some inhuman entity that floats around. In addition, the supporting characters are very likable and important in the storyline this time around; the third actress (playing the allegedly psychotic student) gives one of the best performances in the entire film. Finally, the play on ghostly voices is really great stuff because the strength of the ghost's voice is a direct gauge on the strength of her existence.
The horror elements may not be the scariest of the "Whispering Corridors" series (that nod goes to "Wishing Stairs"), but they are very unique (especially the elevator scene). If you're looking for anything more than a few tame deaths you'll most definitely be disappointed. No matter though, this film is still magnificent anyways.
Fans of Hollywood horror may be disappointed that the teen girls in this movie are actually somewhat mature and likable. I know American audiences tend to like their teens as trashy and stupid as humanly possible, but the girls in Voice are very intelligent and classy. This may prove "boring" for your typical teenie bopper surfer kids who like American slasher movies, which is yet another reason to watch this movie. Oh, and this film does make sense. Check out my explanation thread on the IMDb message board.
In all honesty, the only South Korean horror film that beats "Voice" out is "A Tale of Two Sisters." But then again, that movie beats everything out anyways. :)