Home Button
Choose General > Home Button to set preferences for double-clicking the Home button. You can set iPhone so that double-clicking the Home button goes to the Home screen, Phone Favorites, or iPod.

Set whether double-clicking the Home button shows iPod controls: Turn on Show iPod Controls to display iPod controls when you’re playing music and double-click the Home button. This feature works even when your display is turned off or your iPhone is locked.