When does professor oak calls you saying that there are weird transmissions in the radio tower in Pokemon shiny gold?

In Pokémon Shiny Gold, Professor Oak contacts you about the weird transmissions in the radio tower after you have earned the Cascade Badge from Misty in Cerulean City. This call typically happens when you are progressing through the game's storyline and is a prompt for you to investigate Team Rocket's activities at the Goldenrod Radio Tower. The call serves as a cue to initiate the events surrounding the tower's takeover by Team Rocket.