Antiestablishmentarianism is a political or social doctrine that opposes a nation's conventional power structure, viewing established social, political, or economic institutions as corrupt, repressive, or unjust

The former employee refused to be forced to answer calls at a corporate, for profit, call center. He’s presumably fighting against the establishment by refusing to just follow conventions in every aspect of his life that he can to fight against economic and social oppression by refusing to do what corporate lackies expect or demand of him. That’s the joke.