One Missed Call

The film begins with a young girl being evacuated from a burning hospital. A firefighter asks her if her mother is in there, but she doesn't respond.
Meanwhile, Shelley, a college student, sits outside near her koi pond in her Japanese-styled home and after having a chat over the phone with Leann, she hears her cat making strange noises. Shelley walks towards it but after a quick second, she finds it standing on the other side of the pond. Bewildered by the situation, she suddenly hears strange murmuring in the pond and out of nowhere, a hand emerges and drags Shelley into the pond, drowning her. A red candy then appears on the surface and the same hand proceeds to drag the cat and drowns it as well.
College student Beth Raymond is talking with her friend, Leann, who attended Shelley's funeral. Leann's cellphone rings with a lullaby-like ringtone, and it says it's from Shelley. Leann listens to the voicemail, which is herself screaming. The voicemail is dated three days in the future. Leann begins to experience hallucinations that make her increasingly anxious. While walking home minutes before the time on the voicemail, she calls Beth. Beth runs to get Leann but unfortunately, she arrives at the same time Leann accidentally falls off from an overpass and lands on top of an speeding train, killing her. Although obviously dead, Leann's hand dials a number on her phone.
At Leann's funeral, Beth's roommate, Taylor, is talking about Shelley receiving a strange voicemail before her death, just like Leann had. Brian, Leann's ex-boyfriend, begins to see disturbing apparitions, and leaves the funeral in a rush. Beth catches Brian outside a coffee shop and Brian shows her a voicemail he received the night Leann died, from Leann's phone. Beth realizes the voicemail is dated mere seconds away. An explosion at a nearby construction site sends a rebar through Brian's torso. Brian coughs out a red hard candy and collapses, as Beth stares in horror.
When Beth arrives home, a distraught Taylor is waiting for her. Taylor says she knows she's next, even though she hasn't received a voicemail. Beth assures her that it isn't going to happen, and takes the batteries out of both their cellphones to ensure they can't receive calls. Later that night, Beth and Taylor are awakened by the lullaby ringtone coming from Taylor's battery-less cellphone. Despite having no power source, Taylor's phone screen displays a video message of her death, dated two days in the future.
The next morning, Andrews visits Beth, saying his sister was the one who called Shelley, despite his sister being dead at the time of the call. He also says he traced the voicemail left on his sister's phone and was going to find out more about the person who sent it, Marie Layton. Andrews goes through the local autopsy reports, and finds a report for Ellie Layton, Marie's eldest daughter, who died of an asthma attack. They find records for hospital visits for Ellie and Laurel, who is Ellie's younger sister. Laurel was injured often, which leads Beth to suspect abuse from her mother. As Taylor's time approaches, Beth races to her, and sees Taylor being choked to death by an unseen force. A red hard candy falls out of her mouth. Suddenly, Beth's phone begins to ring and she finds a voicemail dated for tomorrow. In an effort to save herself, Beth decides to find out more about the hospital fire alone.
At the hospital and with less than half an hour left, Beth is frightened by various apparitions. She runs into Andrews, and the two try to escape the hospital. They are separated and Beth finds a crawlspace. She discovers Marie's body, burned to death and clutching a cell phone. At the time of Beth's foretold death, Marie's corpse assaults her while weeping. It is later revealed that Marie was actually protecting Beth, not trying to hurt her. Beth survives her predicted death.
While visiting Laurel, Andrews goes to her room and finds her teddy bear with a video disc in its back. The disc is a video of a camera Marie hid to monitor Laurel and Ellie. The disc reveals that Ellie had cut Laurel's arm with a butcher knife. Marie had entered and realized that the abuse she has been blamed for has been Ellie all along. She then left to take Laurel to the hospital, locking Ellie in the bedroom. Ellie suffers an asthma attack and dies while dialing her mother's cell, making Marie the first real victim of the curse as she died in the hospital fire after leaving the room. Laurel tells Andrews that though Ellie hurt her, she always gave her the red hard candies, the ones found in the mouths of all the victims. Andrews' cellphone begins to ring with a voicemail dated for thirty minutes later.
Andrews realizes that the force behind the murders is Ellie, and races to Beth. After he arrives at her house, the two hear a knock on the door. As Andrews looks through the peephole, a knife stabs through it and kills him. Ellie's spirit appears and reaches out to kill Beth. The spirit of Marie then appears and grabs Ellie, saving Beth yet again. A red candy spills out of Andrews' mouth and his cell begins to dial a number on its own, revealing that Ellie's ghost is still out there, and more people will die.

Question: How did Ellie Layton die?
Answer: Asthma attack