Area code 331 is an overlay for area code 630, serving the western suburbs of Chicago, including cities such as Naperville, Aurora, Wheaton, and Downers Grove. It was introduced in 2007 to address number exhaustion in the rapidly growing suburban corridor west of the city. The 331/630 region is known for its mix of residential communities, corporate headquarters, and educational institutions. Ten-digit dialing is required throughout the area. While 630 retains historical significance, 331 has become increasingly common on mobile and business lines. The dual-code area continues to be one of the most dynamic and affluent regions in Illinois.