In this chapter, a group of men approach Atticus to discuss Tom's trial. They aim to stop him from defending Tom, and to convince Atticus to simply let this case go. Atticus sends Scout and Jem in the house, where they watch the developments from behind the curtains. Things are civil at first, but then the men seem to move to attack.
There was a murmur among the group of men, made more ominous when Atticus moved back to the bottom front step and the men drew nearer to him.
Suddenly Jem screamed, "Atticus, the telephone's ringing!"
The men jumped a little and scattered; they were people we saw every day: merchants, in-town farmers; Dr. Reynolds was there; so was Mr. Avery.
So Jem shouts at Atticus in order to break the tension and prevent any violence. Jem is afraid for Atticus when he sees the men surround him, and shouting that the phone is ringing is his way of throwing the crowd off guard and ending the conversation. When the crowd breaks up, the Finches see that it consists of men they've known their entire lives.. This is one example of the Finch children diffusing tension and acting more rationally than the adults in the novel. The second incident comes at the jail, when Scout interrupts the mob coming to lynch Tom.