Trivia: While the film was always meant to be something of a horror-comedy, director M. Night Shyamalan supervised three versions of the film during editing- one played mostly for horror with little comedy, one played mostly for comedy with the horror downplayed, and one that tried to balance both tones equally. The version that balanced horror and comedy equally is the final version released in theaters.
Trivia: M. Night Shyamalan personally financed the film so that he could have complete creative control over the final product. He wanted to make a low-budget, back-to-basics movie, as he had just finished working on a string of high-budget critical and commercial failures (including "After Earth" and "The Last Airbender") where he had little creative control due to the budgets and studio-mandates. It paid off, as this film, his next film "Split" and his television series "Wayward Pines" were generally considered a "return to form" for Shyamalan by critics.
Answer: 911 calls are usually automatically routed to the nearest local emergency dispatch center. From what I remember (haven't seen the movie in a while, so I could be wrong), the mother was still out of town, so dialing 911 wouldn't have been much help... she'd need to be rerouted, etc. Directly calling the police station was simply a faster way to contact the authorities closest to her children.
TedStixon