After Pryce is defeating and the gym is exited, you will be alerted that Team Rocket has taken over the radio. Sure enough, if you listen to the radio on the Pokegear, the Team Rocket Announcement is playing on every channel. Heal your Pokémon and fly/walk to Goldenrod City.
Storming the Radio Tower
Enter Goldenrod City, which is now littered with Team Rocket Grunts. Go into the underground tunnel on the north side of the city. Walk right to the Rocket Grunt, who will change you into a Team Rocket uniform. Note that you can't leave the city with a uniform on. Enter the radio tower next. Talk to the grunt guarding the stairs. He moves out of the way, but before you can get up, your rival comes by and talks to the grunt. Then he notices you and blows your cover. You end up battling the Rocket Grunt.
After he is defeated, he comments on you being "too powerful" and rushes upstairs. Your rival leaves to battle Lance. Go up the stairs and battle the grunts.
- Arbok (lvl. 26)