Pokemon Red: How to get into the basement?

I'm playing Pokemon Red (Generation I) on my phone, and at Celadon City, I went to Rocket Game Corner and defeated the Rocket guarding the poster hiding the on which supposedly opens the Rocket Hideout leading to Giovanni. Then, when I press A, my character only says, "Hey! A switch behind the poster!? Let's push it!" After which nothing happens. I still can't access the basement. Did I miss out a step?