This actually happened to Anna. A mystery number had been calling her for a very long time and she finally picked up. It was a wrong number. Someone had been calling her for half a year trying to reach someone and they had the wrong number. I theorized that a relationship with a friend or loved one was possibly being slowly destroyed over the course of these months because they appeared to never be taking their calls. Then when possibly confronted in real life, they would deny not taking the calls which would further increase the tensions. Either that or a business relationship that was never allowed to progress.