If the phone has a different width to height ratio than the virtual resolution that you are using, there will be black bars either above and below or left and right.
You can actually draw in those spaces.
Use the command SetScissor(0,0,0,0) after setting your virtual resolution.
To hide the bars on devices that don't have the same width to height ratio, I center my background image (or animation) and create it so that it is larger than my virtual resolution.
You can specify positions with negative X or Y to put an image, text or sprite to the left or above, respectively, the main display. Similarly when the position is past the right or bottom. If a device has a ratio different from your resolution, the image/sprite/text may appear where the black bars would be.
Cheers,
Ancient Lady