Why does Jem say the phone is ringing when it is not in Chapter 15 of To Kill a Mockingbird?

In Chapter 15 of "To Kill a Mockingbird," Mr. Heck Tate knocks on the door of the Finch house.  As Scout observes,

In Maycomb, grown men stood ouside in the front yard for only two reasons:  death and politics.

However, they are present for another reason:  they are uneasy about Tom Robinson who is in the jailhouse and the "Old Sarum bunch" who threaten to come to the jail, according to Mr. Link Deas.  Mr. Deas also says to Atticus,

'--don't see why you touched it in the first place....You've got everything to lose from this, Atticus.  I mean everything.'

'Do you really think so?'

This was Atticus's dangerous question, Scout narrates, whenever he challenges someone's judgment.  This question is met with an ominous murmur in the crowd and the men move in closer to Atticus.  To diffuse the tension, Jem screams, "Atticus, the telephone's ringing!"  When Jem breaks the tension with this cry, the men start and move away.

'Well, answer it son," called Atticus.

The men laugh; they again see Atticus as one of them, and they depart:  "they were people we saw everyday." 

This scene, while innocuous, foreshadows the a truly increasing danger to Atticus as he sits at the jail, alone, and tries to protect Tom Robinson from the mob.  In this scene, Scout diffuses a clearly tense situation.