How to Enter the Bell Tower in Pokémon SoulSilver

Step-by-step walkthrough:

1. Get the Card Key: After defeating Team Rocket in Mahogany Town's Radio Tower, a couple in the Pokémon Center (near stairs) gives it to you.
2. Go to Goldenrod City: Enter the building left of the Pokémon Center's south entrance. Use Card Key on the door. Talk to the man on 2F for the Lift Key.
3. Goldenrod Dept. Store: Take escalator to upper floors. Talk to clerk at elevator top to activate it. Ride to roof. Use Lift Key on door. Talk to old woman for TM49 (Echoed Voice).
4. Goldenrod Radio Tower: Enter, go to 4F. Use TM49 (Echoed Voice) on person by window.
5. Return to Bell Tower: Go back to the locked door left of Pokémon Center south entrance (the one you used Card Key on earlier). Now enter the Bell Tower instead of Radio Tower.

You can now climb the Bell Tower to encounter Ho-Oh (after getting Rainbow Wing).