How does Lee use Calpurnia's actions in dealing with Tim Johnson to show the subtle discrimination prevalent in small Southern towns?

When Calpurnia calls Atticus's office about the rabid dog, she is forced to use subservients language even in the face of an emergency. She uses "proper" etiquette even though Atticus is addressed by his first name by his own children. As well when Calpurnia risks her life to warn the Radley's, they have no phone, Jem notices she knocks on the front door. He points out that, "she's supposed to go around in back." Jem doesn't mean to be offensive, he is just pointing out the racism entrenched in everyday life.