Stupidity: For Sarah's murder to work, a lot of coincidences and unpredictable, convenient behaviour from the victim have to happen; she has to Miss the security guard and not hear them, she has to seek shelter of all places right in the costume room full of effigies of the killer, where the killer himself happens to be posing, she has to be completely inefficient at calling help with her cell phone.
Stupidity: Scream is always considered a parody of the genre; many deaths are grounded in reality, but the one at Jennifer's house it just isn't, since for that to happen the killer needed to know that one of the guests he scared out of the house would have to go inside the kitchen (a different room, further inside) to look at the remaining page, be unaware of the gas and light something to see it, instead of bringing it outside or using a portable flashlight like it happened earlier. None of that was predictable.
Answer: But Roman is in police custody when the call happens and Kincaid has Dewey's phone.