The 331 area code is used for Naperville as well as its neighbor Aurora. As a fairly recent area code, it was implemented as an overlay to the 630 area code in 2007.Currently, the 331 area code is mainly preserved for small suburbs of western Chicago, including a few neighborhoods in the Cook and Dupage Counties.he Naperville area code is fairly wealthy with a median family income of $130,000. Furthermore, only 2.5% of the population in the 331 area code lives below the poverty line.