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Area Code 331 is a relatively recent addition to the North American Numbering Plan, officially introduced in 2012. This overlay area code primarily serves the suburbs of Chicago, Illinois, covering a region that includes parts of DuPage County and other neighboring areas. The establishment of area code 331 was necessitated by the increasing demand for telephone numbers in the densely populated region, which had previously utilized area code 630.

Before 331's introduction, area code 630 had been in service since 1996 and faced significant number exhaustion due to the proliferation of mobile devices, VOIP services, and business lines. An overlay plan was chosen to accommodate new numbers without altering existing area code assignments, allowing residents and businesses to retain their original 630 area code while enabling new phone lines with the addition of 331.

The area code covers a diverse range of communities, including prominent cities such as Naperville, Elmhurst, and Aurora, known for their blend of urban and suburban characteristics. With the overlay system, users in the 331 area must use ten-digit dialing, necessitating both the area code and the local number for all calls, including those within the same area code. This requirement reflects a trend in telecommunications where increased demand for numbers necessitates adjustments in dialing practices.

As telecommunications continue to evolve, area code 331 plays a critical role in meeting the region's connectivity needs, while reflecting broader shifts in technology and consumer behavior.