To open a model #554 or similar wall phone, look at the bottom, and you will find a metal "button" in an opening near where the handset cord comes out. Press on it with a screwdriver, and the cover should come loose, and you can lift it off from the bottom. DON'T press too hard, or you'll bend the metal.
To open a 2554-type wall phone, 1654 rotary-dial Mini-Wall, or many Trimline-type phones, use the end of a paper clip to remove the clear plastic strip that covers the paper strip where the phone number goes. Then remove the paper strip, and you'll see two screw heads. Loosen the screws, and the cover will come off. The screws don't come completely out.