Mobile Detective: Keitai Deka The Movie (2006)

Although they are still teenagers, the Zenigata Sisters are detectives, and have a grandfather in the Inspector General. They possess an uncanny knack for solving crimes, but no one knows that they are investigators. The Zenigata Sisters have been busy solving crimes around the nation, and are faced with a case surrounding their own family. Unless three crimes are solved, the perpetrator threatens to escalate his crimes. It is then that Rui, Mai, and Rei begin receiving mail on their cell phones from the police department. The trap set by the perpetrator has the sisters stumped.