Deliria

Late at night inside a theater, a troupe of actors and crew consisted of the director Peter, Alicia, Mark, Sybil, Betty, Corrine, Laurel, Danny, Brett, and Ferrari are rehearsing a musical about a mass murderer known as the Night Owl. When Alicia sprains her ankle, her and Betty sneak out of rehearsal for medical assistance, the closest being a mental hospital. When speaking to the psychiatrist, Betty notices an imprisoned patient named Irving Wallace, who was a former actor gone insane and committed a killing spree. Unbeknownst to any of them, Wallace killed one of the attendants with a syringe and sneaked out of the asylum to hide inside Betty's car. Upon returning, Peter fires Betty for leaving during the rehearsal. Outside, Betty returns to the car upon Wallace murdering her with a pickaxe. Moments later, Alicia finds her body and contacts the police, resulting the body removed and stationed two officers on the outside premises.
Meanwhile, the director creates an idea by altering the play's script; he renames the show's antagonist to Irving Wallace instead of an ambiguous killer, and insists that everyone stay the night to begin immediate rehearsals with the new material. The group reluctantly agrees to stay with promise of additional cash, and Corrine hides the theater's exit key. Unknown to the group, Wallace has donned the theater's owl costume.
Peter shoots a scene with Corrine, and Wallace appears in the costume to strange her before stabbing her to death. Without the key's whereabouts, the group begins to panic and the killer disconnects the phone lines to prevent them from contacting the officers. While the group tries to find an escape route, Ferrari is stabbed by Wallace and hangs his body upon being found by the group. While Peter and Danny leave the group inside a room and search for the killer, Laurel notices Wallace outside trying to open the door and the group barricades the door; the killer then breaks the window to grab Mark before killing him with a power drill through the door. Peter and Danny return from Mark's murder and they plan to stick together and defend themselves. While Wallace ties up Brett and puts the costume on him, Peter goes up to the upper catwalks despite Alicia being knocked out and accidentally hacks up Brett with an axe, thinking it was Wallace. Soon, Sybil is pulled down by the real killer and she is disemboweled. Danny immediately goes down and is also killed by Wallace with a chainsaw. Cornering Peter, Wallace cuts off his arm containing the axe before the chainsaw runs out of fuel. The killer takes the axe and ultimately decapitates the director.
Alicia wakes up and finds a wounded Laurel hiding in the shower room. While she hides, Wallace grabs Laurel and stabs her before dragging her body. Alicia arms herself and searches the key, only to see Wallace sitting next to the group's bodies placed around the stage and covered with feathers. Underneath the stage, she successfully finds the key defends herself against Wallace before going up to the catwalks. Just as Wallace corners her, she sprays a fire extinguisher into his face and the killer hangs onto a loose cable. After the cable is severed and the killer falls, she makes her way to the door but Wallace attacks again and she dumps a burning bin onto him, igniting him. She then escapes the theater and goes to the police about the events. The next morning, Alicia returns to the theater and finds her missing watch, just before an unmasked Wallace prepares to attack her. Willy shoots him in the head and a disturbed Alicia walks out. As the credits roll, Wallace survives from his headshot and smiles into the camera.

Question: Why does the killer disconnect the phone lines?
Answer: Prevent them from contacting the officers