The closed circle of suspects is a mystery trope that has commonly carried onto the slasher / thriller genre even though it's not necessary. The purpose isn't necessarily to limit suspects, but also to keep the victims within the killing box (Fringe examples might be Phone Booth or Speed ).

In cozy mysteries, this was a narrative device not just to box in the culprit and the victims, but also to make it clear to the reader that this is your set of suspects. (Mysteries traditionally were puzzles that the reader was supposed to be able to follow along and solve with the clues found by the investigator... though the authors didn't always play fair.) The classic example is the bridge between mystery-thrillers and slashers, And Then There Were None by Agatha Christie, in which the trek to the island and an imminent storm really secures the notion that no-one is getting in or out. (Plot point: -- 🤓 -- Vera Elizabeth Claythorne, a PE instructor is quite fit and a strong swimmer, and might have been able to swim to shore and either outrun or weather the storm. She chooses not to, though.)

This whole exercise was started by a supercut of movie instances in which phone service failed, a narrative device to lock in the participants (killers, investigators and victims alike) and lock out anyone else, and this was during that societal transition as people adjusted from often being separated, to always having a connection handy.

My point for the exercise was to note that instant communication may make a circle leaky, but it adds bunches of cool new tropes, and doesn't require turning off the phones (or the prior murderer trick, cutting the house phone lines.)

I have a different rant about the police, who, in mysteries go from clever and central to the solution to totally useless without the investigator. But in the 21st century, they can also turn your mystery into a dystopian horror as they SWAT into your home, kill all the minorities (and the dogs) and arrest everyone else for the homicides they [the police] committed.