The only thing is that a very similar thing happened to us - only it wasn't a prank, it was an instance of pocket dialing.
My son, who was 19 at the time, was here at our house with his buddy playing Xbox. The game is played LIVE online with other players from around the world. The players wear headsets and talk to each other while they are playing. In this particular game, my son and his buddy were on the same team playing against another team.
More background: Grandma knew that we were out of town and the our son and his buddy were at the house watching the house while we were away.
At some very critical point in the game my son accidentally pocket dialed his Grandmother. He didn't know he had called her and she didn't know he was playing Xbox live. All she could hear were muffled yells for help. She could hear "HELP" and "They're all over me - Get over here! I'm being attacked! Dude!" General stuff like that.
She was horrified and didn't really know what to do so she called the direct line for the Sheriff's department which is just a mile away from our house. The Sheriff came zipping up to the house only to discover two teens playing Xbox. Fortunately for the teens there was no beer involved or this story could have had a different ending. But they were just innocently playing a video game and having some fun.
The Sheriff was really nice and called Grandma back and explained pocket dialing to her.
It is a cautionary tale on so many levels - but it can happen.
So, perhaps, before you start to play Call of Duty you should put your cell phone in a safe place across the room, make sure the windows are shut so the neighbors don't hear you yelling, and make sure that if you are underage you are not drinking beer while your parents are out of town.