Let’s say, hypothetically, you had a mate who’d had some ‘work done’ i.e. as he got older he’d gone knock, knock knockin on a few surgeons doors. And you said the following joke to him:

“John Travolta called; he wants his face back.”

On the surface this is simply connecting John Travolta’s own plastic surgery exploits to your friends; but deeper, below the surface, there is another reference being made: to the movie John Travolta starred in with Nicolas Cage – where they swapped faces, called ‘Face off’. With this knowledge you drop from one level to another, and the deeper level is funny (not just insulting) as has more thought to it.