I know that your rival's sister calls you, but when does she start calling you, also, do you get an item that allows you to call someone? If so, when?
Mass outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex and has entered the Hall of Fame, a "swarm of Pokémon" will appear every day in a particular location, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events. Information about mass outbreaks can be found in Sandgem Town by talking to Lucas/Dawn's sister or by watching the Sinnoh television. Source: https://bulbapedia.bulbagarden.net/wiki/Mass_outbreak I hope I've helped!
