Area Code 331
Illinois
Area code 331 serves Illinois, primarily Aurora, in the Central Time (CT) time zone. It is an overlay area code sharing territory with area code 630.
Overlay Area Code
This area code is part of an overlay region, meaning the same geographic area is served by multiple area codes. Overlay codes were introduced to meet growing demand for phone numbers without requiring existing customers to change their numbers.
This area code shares its geographic region with: area code 630 (Aurora).
In service since: 07-Oct-2007
Historical Timeline of Area Code 331
Area code 331 was assigned by NANPA on July 23, 1999, and went into service on October 7, 2007, as an all-services overlay on area code 630 covering Chicago's western suburbs. The more-than-eight-year gap between assignment and activation was driven by mandatory number-pooling conservation that extended 630's capacity. 331 was part of a simultaneous overlay batch that also assigned area code 464 (overlaying 708) and area code 872 (overlaying 312/773), all under Illinois Commerce Commission Docket 98-0847. Source: NANPA PL-195
Key milestones:
- 1947: Area code 312 created as one of the original 86 North American area codes, serving Chicago and northeastern Illinois. According to NANPA records.
- 1989: Area code 708 split from 312 on November 11, 1989, taking Chicago's inner suburbs. According to NANPA records.
- 1996: Area code 630 entered service on August 3, 1996, split from 708 to cover DuPage and most of Kane counties. Mandatory 1+10 digit dialing took effect November 30, 1996. Source: Wikipedia
- 1999: NANPA assigned 331 on July 23, 1999, under PL-195, based on ICC Docket 98-0847 approval. The assignment required mandatory thousand-block number pooling to be implemented in 630 before 331 activations could begin. Source: NANPA PL-195
- 2007: Area code 331 entered service on October 7, 2007, as an all-services overlay on 630. Mandatory 1+10 digit dialing was required across the 630/331 region. Source: Wikipedia
The 630/331 overlay complex serves Chicago's western suburbs across DuPage, Kane, and parts of surrounding counties, covering communities including Aurora, Naperville, Wheaton, and Downers Grove. NANPA projects no exhaustion of the complex until Q2 2066. Source: NANPA
Unique Facts About Area Code 331
Among Illinois area codes, 331 occupies a unique position. Source: NANPA PL-195
- Longest activation gap in Illinois: Area code 331 had more than eight years between NANPA assignment (July 23, 1999) and activation (October 7, 2007), one of the longest delays for any Illinois area code. The gap resulted from mandatory thousand-block number pooling requirements that extended area code 630's useful life. Source: NANPA PL-195
- Part of a four-overlay batch: NANPA PL-195 assigned area code 331, area code 464, and area code 872 simultaneously on October 24, 1999, as overlays for 630, 708, and 312/773 respectively, all under Illinois Commerce Commission Docket 98-0847. Source: NANPA PL-195
- No projected exhaustion until 2066: As of 2025, NANPA projects no exhaustion of the 630/331 overlay complex until Q2 2066, providing decades of remaining numbering capacity for Chicago's western suburbs. Source: NANPA
- Number pooling conservation: The eight-year delay before 331's activation was caused by the FCC's mandatory thousand-block number pooling program, which required carriers to implement pooling in 630 before new area code assignments could begin. This conservation measure significantly extended 630's operational life. Source: FCC 00-429
FCC Regulatory History
Area code 331 was governed by three key regulatory actions: Illinois Commerce Commission Docket 98-0847 approved the overlay in 1999, NANPA PL-195 formally assigned the code, and FCC 00-429 established the numbering framework for its implementation. Source: NANPA PL-195
| Year | Event | Docket | Regulatory Action & Impact |
|---|---|---|---|
| 1999 | Overlay Approval | ICC 98-0847 | Illinois Commerce Commission approved all-services overlay for 630 (331), along with overlays for 312/773 (872) and 708 (464). Mandated number pooling implementation before new code activation. Source: ICC |
| 1999 | Code Assignment | PL-195 | NANPA assigned NPA 331 to overlay NPA 630 based on ICC Docket 98-0847 approval. Start dates to be determined based on number pooling activation and its impact on extending existing area code life. Source: NANPA PL-195 |
| 2000 | Numbering Framework | FCC 00-429 | FCC established 60% utilization threshold for carriers, mandatory 10-digit dialing in overlay regions, and comprehensive audit program. This order governed the numbering framework under which 331 was implemented. Source: FCC 00-429 |
Demographics & Economics
- Total Population
- 2,126,454
- Population Density
- 1,231 people/sq miabove U.S. average (U.S.: 94 people/sq mi)
- Median Household Income
- $110,474above U.S. median (U.S.: $78,538)
- Poverty Rate
- 7.1%below U.S. average (U.S.: 12.4%)
- Unemployment Rate
- 4.8%near U.S. average (U.S.: 5.2%)
- Education Attainment
- Bachelor's Degree+44.4%above U.S. average (U.S.: 35.0%)High School or Lower25.3%
Statistics are population-weighted aggregations from Census ACS 5-year estimates.See Sources for details.
Geographic Coverage
State
IL
Major Cities
- Aurora
- Naperville
- Bolingbrook
- Wheaton
- Downers Grove
- Plainfield
- Elmhurst
- Lombard
- Romeoville
- Bartlett
- Carol Stream
- Streamwood
- Hanover Park
- Addison
- Oswego
- Woodridge
- Glendale Heights
- St. Charles
- Glen Ellyn
- Batavia
- West Chicago
- Westmont
- South Elgin
- Lisle
and 37 more cities
Time Zone
- Central Time (CT)
Current Time
County Coverage
Population-weighted county share based on aggregated ZCTA population (100% = total mapped population for area code 331).
| County | Population | Share |
|---|---|---|
| DuPage, IL | 1,049,109 | 49.3% |
| Kane, IL | 437,352 | 20.6% |
| Will, IL | 276,330 | 13.0% |
| Kendall, IL | 232,928 | 11.0% |
| Cook, IL | 129,147 | 6.1% |
| DeKalb, IL | 1,588 | 0.1% |
Call Context
This area code serves a major metropolitan region with high population density in IL. The primary city is Aurora.
Nearby Area Codes
Area codes geographically closest to 331.
Other Area Codes in Illinois
Area codes that also serve Illinois, linked for easy comparison.
Frequently Asked Questions
Common questions about area code 331 and how phone numbering works.
Assigned July 23, 1999; in service October 7, 2007. One of the longest gaps between assignment and activation in Illinois, due to number pooling conservation extending 630's life.
As of Q1 2025, NANPA projects no exhaustion of the 630/331 complex until Q2 2066, a significant extension from prior forecasts.
Phone numbers assigned to area code 331 are tied to where they were originally provisioned — not the caller's current location. This code serving roughly 2126k people covers Aurora in IL, so a number originally set up there will retain the 331 prefix even if the owner moves or uses VoIP. As a densely populated metro area, Aurora sees high call volume from both local and redirected numbers. Mobile phones keep their original number when users relocate, and businesses often adopt local area codes to establish a regional presence.
Start by looking up the area code on this site to understand its geographic context — area code 331 is associated with Aurora in IL. For caller-specific details, use your carrier's caller ID features or check public directories. If you suspect fraud, you can report it to the FTC. To file a complaint about robocalls or spoofed numbers, visit the FCC Consumer Complaint Center. You can also register with the National Do Not Call Registry.
When an area's available number pool runs low, regulators introduce an "overlay" code covering the same territory. Both codes coexist, and new numbers are assigned from whichever has availability. Area code 331 serves the Aurora region in IL, where growing demand for phone numbers necessitated an additional code. This area code shares its region with area code 630. Learn more in our overlay vs split guide.