The incident with Tim Johnson, the rabid dog, occurs in Chapter 10 of the novel. At this point, the children are lamenting Atticus's "old age" and desperately need him to do something to prove that he is not ancient or feeble. Lee mainly includes the anecdote about the dog to illustrate the children's initial change in the way they view their father.
However, the author also includes Calpurnia in the scene not only to show her protective, motherly role in Jem's and Scout's lives, but also to demonstrate how even in small incidents Calpurnia is treated differently. First, when Calpurnia calls Atticus's office, she refers to him as "Mr. Finch" and "yessir." While this isher normal manner of addressing him, it does stand out to the reader that even in a time of crisis, she follows "proper" etiquette, despite the fact that Atticus's own children call him by his first name. Calpurnia has been conditioned by her small town to address white people carefully, and at this point in her life it is second nature to her.
Secondly, when Calpurnia talks to the operator to get her to warn other residents on the Finches' street, the operator does not at first believe her. She has to waste valuable time pacifying Miss Eula, the telephone operator, before she can hang up.
Finally, when Calpurnia realizes that the Radleys do not have a phone, she risks her life to run outside--where the rabid dog is advancing--and knock on the Radleys' door. When Scout sees Calpurnia at the Radley house, she says,
" 'She's supposed to go around in back' " (124).
Scout, just like other children in her town, practices the discrimination of the town even though her father does not teach or condone it.