ATM

David Hargrove (Brian Geraghty) is an accountant having trouble asking out his co-worker Emily Brandt (Alice Eve), who is resigning for a position with the MPO. At a Christmas party, David offers to drop Emily home. Reluctantly, David also agrees to drive home his other co-worker Corey Thompson (Josh Peck), who accompanies them. During the car ride at midnight, Corey forces David to stop at a local ATM booth, needing to withdraw cash to buy pizza at a restaurant.
David spites Corey by parking some distance away, making him walk. However, David and Emily join Corey inside when he encounters card problems. When the three are about to leave, they spot a hooded figure (Mike O'Brian) in a parka coat lurking outside in the parking lot. They panic and Corey suggests they return to the car, but David and Emily suspect the figure to be a robber. They also discover he cannot enter without an ATM card, though other circumstances soon prove this wrong.
When the hooded man kills a dog walker, they attempt to phone the police, only to realize that their cellphones are left in the car. The hooded man shuts down the booth's heater, rendering the booth cold to match the weather. David opts to negotiate their safety by giving the killer $500, earrings and a watch. He uses this to escape to his car where he finds out the wires are severed, and the car cannot start. David attempts to call 911 but is attacked by the pursuing figure. He accidentally drops the phone midway, in an attempt to escape from the hooded man.
Trying to lure assistance, Emily then uses her lipstick to write "HELP" on the booth's window, as the hooded figure tidies the scene outside. David, Emily and Corey are freezing when a security guard (Ernesto Griffith) locates them. When the guard tries to call the cops, the hooded man kills him using a tire iron from David's car trunk, leaving the three witnesses in shock. When a man with a similar coat abruptly enters the booth, he is subdued and killed by David and Corey, but is later revealed to be only an innocent janitor. Frustrated, Corey takes the janitor's coat and leaves, but gets caught and stabbed by the hooded man.
After several hours, David and Emily realize Corey is still alive. They retrieve him from outside, narrowly managing to return to the booth before the hooded man can get to them. The hooded man then blocks the booth door with David's car and tries to freeze them to death, by filling the booth with cold water. Corey eventually dies of blood loss and hypothermia. David elevates Emily on his shoulders to successfully trigger the fire sprinkler system alarm using Corey's lighter, a trash can, and some paper; but Emily falls and dies.
Relentless, the hooded man sends the security guard's car into David's car which slams into the booth and breaks it down. Angered, David improvises a Molotov cocktail and throws it at the hooded killer, but the figure he sets ablaze turns out to be the dead security guard in a parka coat. The police arrive, and David is arrested as the hooded man conceals himself. As David is driven away, he sees many unknown figures wearing parka coats in the crowd at the crime scene. The killer among the crowd, spies on David and leaves the scene. The police recover surveillance recordings of the events within the ATM booth, but it is made clear the killer had planned his actions so as not to appear in the footage, effectively framing David for the killer's crimes. The killer has also planted other evidence against David. In the final scene of the film, the hooded man, who remains unknown, returns to his headquarters, where he begins to map out his similar attacks on a mini-mart, a farm house, and other ATMs.

Question: Who tried to call the cops?
Answer: The guard