331 Area Code: Chicago, IL | Location & Time Zone

The 331 area code serves Chicago’s thriving western suburbs in Illinois, covering major cities including Aurora, Naperville, and dozens of vibrant communities across DuPage, Kane, Cook, Will, and Kendall counties. Introduced in October 2007 as an overlay to the existing 630 area code, the 331 area code ensures continued telephone service availability for one of the fastest-growing regions in the Chicago metropolitan area.

What is the 331 Area Code?

The 331 area code is a telephone numbering plan area that serves the western suburbs of Chicago, Illinois. This area code was implemented on October 7, 2007, by the North American Numbering Plan Administration (NANPA) as an overlay to the existing 630 area code. As an overlay area code, 331 serves the same geographic region as 630, requiring ten-digit dialing for all calls within the coverage area.

The implementation of the 331 area code was necessary due to the rapid population and business growth in Chicago’s western suburbs, which created increased demand for telephone numbers. The region now serves a combined population of over 7.5 million residents, making it one of the most populous area code regions in Illinois.

331 Area Code Location & Coverage

Cities and Regions Covered

The 331 area code covers portions of DuPage, Cook, Kane, Will, and Kendall counties. Major cities within the 331 area code include Aurora (Illinois’s second-largest city), Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Glen Ellyn, Lombard, Carol Stream, and Bartlett.

Additional communities served by the 331 area code include Addison, Batavia, Bensenville, Bloomingdale, Darien, Glendale Heights, Hanover Park, Lisle, North Aurora, Roselle, Saint Charles, Streamwood, Villa Park, West Chicago, Westmont, and Woodridge. The coverage area also extends to smaller suburban communities such as Argonne, Big Rock, Bristol, Burr Ridge, Clarendon Hills, Elburn, Eola, Fox Valley, Geneva, Hinsdale, Itasca, Keeneyville, Lemont, and Wayne.

Geographic Coverage Map

The 331 area code coverage area spans approximately 1,200 square miles of prime suburban Chicago territory. The region extends from the Cook County border in the east to Kane County communities in the west, encompassing the heart of DuPage County and northern portions of Will County. This strategic location provides residents and businesses with easy access to downtown Chicago via major highways including Interstate 88, Interstate 355, and the Illinois Tollway system.

Time Zone Information

The 331 area code operates within the Central Time Zone, observing Central Standard Time (CST) during winter months and Central Daylight Time (CDT) during daylight saving time from early March to early November. This time zone alignment with the greater Chicago metropolitan area ensures seamless business and personal communications throughout the region.

331 Area Code Details

History of 331 Area Code

When Was 331 Created?

The 331 area code was officially put into service on October 7, 2007, making it the 333rd area code implemented in North America and the 12th area code assigned to Illinois. The area code was created to address the growing shortage of available telephone numbers in the 630 area code region, which was experiencing rapid population and economic growth.

The creation of area code 331 was part of a comprehensive numbering resource management strategy implemented by the Illinois Commerce Commission and NANPA to ensure continued telephone service availability without disrupting existing 630 customers. The first telephone number prefix in the 331 area code was assigned on July 6, 2007, just months before the area code officially went into service.

Area Code Changes and Overlays

Area code 331 was introduced as an overlay to the existing 630 area code, which had been serving the region since 1996. Unlike area code splits that divide geographic regions, the overlay system allows both 630 and 331 numbers to coexist within the same geographic boundaries. This approach minimized disruption for existing customers while providing additional numbering resources for new residents and businesses.

The implementation required the introduction of mandatory ten-digit dialing for all calls within the region, including local calls. Currently, 203 active prefixes are assigned within the 331 area code, with 49.8% designated for landline services, 49.8% for wireless services, and 0.49% for other telecommunications services.

How to Call 331 Area Code

Dialing Within the Area Code

For local calls within the 331 area code region, you must use ten-digit dialing format: 331-XXX-XXXX. Even when calling another number within the same area code, the full ten digits including the 331 area code must be dialed. This requirement also applies when calling between 331 and 630 numbers within the same geographic area.

Dialing From Other Area Codes

When calling a 331 number from outside the local area code region within the United States, dial: 1-331-XXX-XXXX. The “1” prefix indicates a long-distance call, followed by the complete ten-digit number including the 331 area code.

International Dialing

For international calls to 331 area code numbers, use the format: +1-331-XXX-XXXX. The “+1” represents the United States country code, followed by the complete ten-digit telephone number. International callers should check with their service provider for specific dialing instructions and applicable international calling rates.

Nearby Area Codes

Population & Demographics

The 331 area code region serves a population of approximately 7.5 million residents when combined with its overlay partner, the 630 area code. The western suburbs covered by area code 331 represent some of the most affluent and fastest-growing communities in the Chicago metropolitan area.

Major economic centers within the 331 area code include Aurora, which serves as a significant manufacturing and logistics hub, and Naperville, consistently ranked among the best places to live in America. The region hosts numerous corporate headquarters, technology companies, and healthcare systems, contributing to a diverse and robust local economy. Educational institutions within the area code include Northern Illinois University’s satellite campuses, College of DuPage, and Waubonsee Community College.

The demographic profile of the 331 area code region reflects a highly educated, diverse population with above-average household incomes. The area attracts young professionals and families seeking high-quality schools, safe communities, and convenient access to Chicago’s business district and cultural amenities.

Frequently Asked Questions

Do I need to dial the area code for local calls in the 331 area?

Yes, ten-digit dialing is mandatory for all calls within the 331 area code region. You must dial the full 331-XXX-XXXX format even for local calls within the same area code or when calling the overlay 630 area code.

What’s the difference between 331 and 630 area codes?

Both 331 and 630 area codes serve the exact same geographic region in Chicago’s western suburbs. Area code 630 was the original area code established in 1996, while 331 was added in 2007 as an overlay to provide additional telephone numbers for the growing population and business community.

When was the 331 area code introduced?

The 331 area code was officially implemented on October 7, 2007, as an overlay to the existing 630 area code to address the increasing demand for telephone numbers in Chicago’s western suburbs.

What cities does the 331 area code cover?

The 331 area code covers major cities including Aurora, Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Lombard, Bartlett, and dozens of other communities across DuPage, Kane, Cook, Will, and Kendall counties in Illinois.

Is 331 a toll-free area code?

No, 331 is not a toll-free area code. It’s a geographic area code serving Chicago’s western suburbs. Calls to 331 numbers may incur long-distance charges depending on your calling plan and location.

Can I keep my 630 number if I move within the area?

Yes, since both 630 and 331 serve the same geographic region, you can typically keep your existing 630 number when moving within the coverage area. Contact your telephone service provider to confirm number portability for your specific situation.

Conclusion

The 331 area code serves as a vital telecommunications resource for Chicago’s dynamic western suburbs, supporting millions of residents and thousands of businesses across five counties in Illinois. Whether you’re planning to move to cities like Aurora or Naperville, establishing business operations in the region, or simply need to understand the calling requirements, the 331 area code represents one of the most important numbering plan areas in the Chicago metropolitan region.

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