331 Area Code: Quick Overview, Location, Map, Scams
12 min read
What is the 331 Area Code? Time Zone and Location
- Area code 331 is located in Illinois and covers Aurora, Naperville, Bolingbrook, and Wheaton — it is an overlay for area code 630 and serves the same area.
- Combined with overlay 630, it covers 33 cities across 5 counties in the western Chicago suburbs, serving a population of 7,553,164.
- The 331 area code serves Chicago's thriving western suburbs, covering major cities including Aurora, Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Glen Ellyn, Lombard, Carol Stream, and Bartlett, as well as Addison, Batavia, Bensenville, Bloomingdale, Darien, Glendale Heights, Hanover Park, Lisle, North Aurora, Roselle, Saint Charles, Streamwood, Villa Park, West Chicago, Westmont, and Woodridge.
- The 331 area code is located in the Central Time Zone — following Central Standard Time (CST, UTC−6) during standard time and Central Daylight Time (CDT, UTC−5) during daylight saving time from March through November.
- Area code 331 was put into service on October 7, 2007 and was the 333rd area code in service — one of 3 area codes introduced in 2007 — created to relieve the pressure on the 630 area code as numbers in 630 were being exhausted by the explosive growth of Chicago's western suburbs.
- 10-digit dialing is mandatory for all local calls within the 630/331 overlay complex. You can format numbers into E.164 and dialable formats using our free Phone Number Formatter.
| Field | Details |
|---|---|
| Area Code | 331 |
| Country | United States |
| State / Region | Illinois (western Chicago suburbs — DuPage, Kane, Will, Kendall, and Cook counties) |
| Area Code Type | Overlay area code (shares region with area code 630) |
| Overlay Codes | 630 |
| Major Cities Covered | Aurora, Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Glen Ellyn, Lombard, Carol Stream, Bartlett, Addison, Batavia, Lisle, Hanover Park, Bloomingdale, Roselle, Saint Charles, Streamwood, Villa Park, West Chicago, Westmont, Woodridge, Glendale Heights, Darien, Geneva, Yorkville, Oswego |
| Primary City Association | Aurora, Illinois (Illinois's second-largest city) / Naperville, Illinois (DuPage County seat) |
| Geographic Coverage | 33 cities across 5 counties in the western Chicago suburbs, including DuPage, Kane, Kendall, Cook, and Will counties — covering approximately 1,200 square miles of the Chicago metropolitan area's western corridor |
| Time Zone | Central Time Zone (CT) |
| Standard Time Offset | UTC −6 hours |
| Daylight Saving Time | UTC −5 hours (CDT) |
| DST Period (2026 Example) | March → November |
| Introduced / Activation Date | October 7, 2007 — the 333rd area code in service and the 12th area code in Illinois |
| Reason for Creation | Created as an overlay to the 630 area code in 2007, as the rapid population and business growth in Chicago's western suburbs created increased demand for telephone numbers that exhausted available supply in 630 |
| Dialing Requirement | 10-digit dialing mandatory (due to 630/331 overlay complex) |
| Coverage Nature | Overlay complex — 331 and 630 share the same western Chicago suburban geography |
| Population Coverage | Combined with overlay 630, serves a population of 7,553,164 across the western Chicago metro suburbs |
| Nearby / Related Area Codes | 224/847 (Elgin, IL), 309/861 (Peoria, IL), 464/708 (Cicero/south Chicago suburbs, IL), 630 (331's parent overlay), 773 (Chicago, IL), 779/815 (Rockford, IL), 872/312 (Chicago, IL). See also other major metro guides like the 440 Area Code, 405 Area Code, and 908 Area Code. |
| Telecom System | North American Numbering Plan (NANP) |
| Major Carriers | 64% of 331 prefixes are assigned to wireless carriers, with 31% to landlines and 5% unknown — reflecting the western suburbs' high mobile adoption rate; T-Mobile, AT&T Mobility, Verizon Wireless, and Sprint are primary wireless carriers |
| Example Phone Format | (331) XXX-XXXX |
| Usage Type | Mobile, landline, VoIP numbers — primarily mobile (64% wireless assignment) |
| Business Use | Highly valuable for local presence across the western Chicago suburbs — home to Naperville (one of America's most consistently top-ranked cities to live and work) and Aurora (Illinois's second-largest city) |
| Number Availability | Currently uses 209 prefixes, assigned from 331-200-XXXX through 331-999-XXXX — with 73% of prefixes still unassigned, ensuring strong long-term availability |
| Area Code Status | Active and growing — with 64% wireless prefix assignment, 331 is predominantly a mobile number code for the western suburbs |
| Common Legitimate Callers | The Joint Commission surveyors, Fermilab researchers, and Edward-Elmhurst Health schedulers are among the most recognized legitimate callers from the 331/630 western Chicago suburbs overlay complex |
| Economic Profile | Naperville is consistently ranked among the wealthiest and most livable cities in the United States. DuPage County boasts a median household income exceeding $90,000, with Aurora serving as a major manufacturing, healthcare, and education hub as Illinois's second-largest city |
| Notable Employers | Fermilab (the world-renowned particle physics laboratory in Batavia), Edward-Elmhurst Health, Advocate Aurora Health, McDonald's headquarters (Oak Brook), Navistar International, and Nicor Gas are among the major employers in the 331/630 corridor |
Is the 331 Area Code a Scam?
| Field | Details |
|---|---|
| Spam Ranking | Ranked #255 for spam complaints nationally, accounting for approximately 0.1453% of all spam complaints |
| Most Common Complaint Type | Prerecorded voice accounts for 54.06% of all reported Illinois spam calls — the dominant statewide complaint type — reflecting the heavy use of automated robocalls across the 331 region |
| Top Reported Cities for Complaints | Chicago, IL leads with 4.92% of complaints, followed by Aurora, IL (1.4%), Island Lake, IL (1.34%), Naperville, IL (0.95%), and Wheaton, IL (0.9%) |
| Peak Complaint Period | January 2026 saw the highest volume of complaints for area code 331, with February 2026 showing no change compared to January 2026 |
| Spam Growth | Spam calls in Illinois increased by 320.1% over the past year — one of the sharpest statewide increases in the country — with the highest volume recorded in September 2025, accounting for 12.33% of all calls |
| Illinois Statewide Fraud Context | Illinois residents filed 102,690 unwanted call complaints in 2024 — with medical and prescription schemes leading at 7,908 complaints, followed by imposter scams (7,507) and debt reduction (7,050) — with over 56,038 of those complaints involving automated robocalls |
| Common Labels | Fake 'judicial services' HR threat caller, misleading health survey robocaller, Nicor Gas/Apple Pay impersonator, 'Can You Hear Me?' voice signature phisher, government agency impersonator, relentless burst caller |
| Spoofing Risk | Scammers spoof 331 numbers to capitalize on its association with affluent Chicago-area communities — a call from a 331 number carries the implicit credibility of Naperville and the western suburb corridor, making recipients more likely to answer. Verify unknown callers using our Phone Number Validator. |
| 'Affluent Suburb' Exploitation | The 331 area code's association with DuPage County's high-income communities makes it particularly valuable for scammers — fake legal complaint calls specifically designed to embarrass victims at work are heavily reported, exploiting the professional nature of the western suburbs' predominantly corporate and healthcare workforce |
| Typical Strategy | Neighbor spoofing — faking familiar Aurora, Naperville, or Wheaton numbers to build trust among affluent western suburb residents before making fake judicial threat, health survey, or utility impersonation demands |
| Signature Scam — 'Judicial Services' Fake Legal Complaint / HR Threat | Scammers call claiming you have a 'complaint on file' that needs immediate attention — identifying themselves as being from 'judicial services' and specifically threatening to report the vague complaint directly to your job's HR or payroll department if you do not update your contact information, using the professional pride and workplace reputation concerns of the western suburbs' corporate workforce as leverage |
| Signature Scam — Misleading Health Survey Burst Calling | The 331 overlay is specifically used for misleading health surveys and relentless bursts of calls designed to wear down resistance — scammers call repeatedly in rapid succession from different 331 numbers, using fake medical or wellness surveys to extract personal health information, insurance details, and Medicare numbers |
| Signature Scam — Nicor Gas / Apple Pay Impersonation | Callers impersonate Nicor Gas — the primary natural gas utility serving the western Chicago suburbs — demanding immediate payment via Apple Pay or prepaid card to avoid service disconnection, exploiting the near-universal reliance on Nicor Gas across DuPage and Kane county households |
| Signature Scam — 'Can You Hear Me?' Voice Signature Capture | Callers open with 'Can you hear me?' or 'Are you the homeowner?' to record your 'yes' response — using it as fraudulent voice authorization for charges to your credit card or phone bill, a tactic exclusively used by scammers that legitimate callers in the healthcare and research-heavy western suburbs community never employ |
| Naperville Police Warning | Naperville police have specifically warned that scammers posing as Dish Network, Payday Loan companies, and government agencies — including fake City of Naperville representatives — call residents claiming they owe back taxes and threatening arrest or large fines if they do not provide credit card or banking information immediately |
| City of Naperville Clarification | The City of Naperville emphasizes that calls from the City regarding utility accounts will show up on caller ID as 'City of Naperville' and almost always show the number 630-420-6059 — the city does not make calls from 800 numbers or other area codes requesting payments for utility services |
| Reporting Authority | Report suspicious calls to the FTC at ReportFraud.ftc.gov, to the Illinois AG's Consumer Protection Division at illinoisattorneygeneral.gov (or call 1-800-386-5438); for Nicor Gas scams, call 1-888-NICOR4U (1-888-642-6748) |
| Key Risk Indicator | 'Judicial services' callers threatening to contact your HR department about vague complaints, rapid-fire health survey calls from multiple 331 numbers, Nicor Gas calls demanding Apple Pay payment to avoid gas shutoff, or calls opening with 'Can you hear me?' from unknown numbers |
| Safety Tip | The City of Naperville and Nicor Gas will never call demanding immediate payment through Apple Pay, gift cards, or prepaid debit cards — and no legitimate process server or judicial agency calls HR departments to embarrass employees. Always hang up and call the utility's official number to verify your account status before taking any action. Set up a clear greeting with our Voicemail Greeting Generator to filter inbound callers. |
What Are the Business Benefits of the 331 Area Code?
| Field | Details |
|---|---|
| Local Presence | A 331 number establishes immediate credibility in Chicago's affluent western suburbs — home to Naperville (one of America's most consistently top-ranked cities for quality of life) and Aurora (Illinois's second-largest city), representing one of the most commercially significant suburban corridors in the Midwest. Pair with an AI receptionist to capture every local lead 24/7. |
| Customer Trust Factor | A local 331 number dramatically increases answer rates vs. unknown out-of-state numbers in a sophisticated, high-income market where local identity and professional credibility are key drivers of consumer and business trust |
| Coverage Advantage | One number covers 33 cities across 5 counties in the western Chicago suburbs, spanning from Aurora and Geneva along the Fox River corridor through the DuPage County powerhouses of Naperville, Wheaton, and Downers Grove to the Will County growth communities of Bolingbrook and Woodridge |
| VoIP Compatibility | Easily available for SaaS platforms, cloud PBX systems, and remote sales teams serving the western Chicago suburban market. Calculate team capacity with our Erlang C Staffing Calculator. |
| Scalability | Covers roughly 2.09 million unique phone numbers — with 73% of 331 prefixes still unassigned, ensuring exceptional long-term availability and making 331 the most available number resource in the Chicago western suburbs |
| Research & Innovation Corridor | Fermilab — the world-renowned particle physics laboratory — is located in Batavia within the 331 coverage area, anchoring a significant scientific research and technology ecosystem across DuPage and Kane counties |
| McDonald's Corporate Corridor | McDonald's global headquarters is located in Oak Brook within the 331 coverage area — along with numerous major corporate headquarters across DuPage County — making 331 a strategically valuable code for any business serving the western suburbs' dense corporate campus ecosystem |
| Marketing Use | Ideal for local SEO, digital campaigns, and customer outreach across Aurora, Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Glen Ellyn, Lombard, Carol Stream, Bartlett, Lisle, Geneva, Saint Charles, Batavia, Woodridge, and surrounding communities. Track acquisition efficiency with our Customer Acquisition Cost Calculator. |
| Startup Friendly | Highly attractive for healthcare IT, research technology, logistics, and consumer goods startups given the region's extraordinary combination of DuPage County's high-income consumer base, Fermilab's research ecosystem, Edward-Elmhurst Health's medical network, and McDonald's corporate campus. Project revenue trajectories using our Sales Growth Calculator. |
| Call Routing Flexibility | Can route calls globally while appearing local to western Chicago suburb customers — particularly valuable for healthcare, corporate services, and technology companies managing Chicagoland and national operations |
| Top Industries | Healthcare (Edward-Elmhurst Health, Advocate Aurora Health, DuPage Medical Group), corporate headquarters (McDonald's, Navistar International, Anixter International), scientific research (Fermilab), utilities (Nicor Gas, ComEd), education (College of DuPage, North Central College, Benedictine University, Aurora University), manufacturing, logistics, and retail benefit most from a local 331 presence |
| Use Case Examples | AI receptionist, corporate campus support lines, healthcare practice and hospital network lines, scientific research and laboratory services, university admissions and affiliated services, logistics and manufacturing supplier support, real estate agencies (DuPage County luxury market), restaurant corporate lines, and local retail and professional services |
Frequently Asked Questions (FAQs): What is the 331 Area Code?
What is the 331 area code?
The 331 area code is a telephone area code in Illinois, United States. It serves the western suburbs of Chicago and works as an overlay for the 630 area code.
Where is the 331 area code located?
The 331 area code is located in northeastern Illinois. It mainly covers communities in the western and southwestern parts of the Chicago metropolitan area, spanning DuPage, Kane, Kendall, Will, and Cook counties.
Which cities use the 331 area code?
The 331 area code is commonly used in Aurora, Naperville, Wheaton, Downers Grove, Elmhurst, Lombard, Glen Ellyn, West Chicago, Bolingbrook, Plainfield, Oswego, Yorkville, and nearby communities.
Is 331 a Chicago area code?
The 331 area code is not used for central Chicago itself (which uses 312 and 773). It is mainly associated with the western suburbs of the Chicago metropolitan area.
What time zone is the 331 area code in?
The 331 area code is in the Central Time Zone (CT). This region observes Central Standard Time (CST, UTC−6) and Central Daylight Time (CDT, UTC−5) during Daylight Saving Time.
Is the 331 area code a scam?
No, the 331 area code itself is not a scam. It is a legitimate North American Numbering Plan area code in Illinois. However, scammers can spoof any area code, including 331, to make calls look local. Be cautious with unknown callers asking for personal, financial, or verification information, and check suspicious numbers with our Phone Number Validator.
Is 331 a toll-free area code?
No, 331 is not a toll-free area code. Toll-free area codes include 800, 888, 877, 866, 855, 844, and 833.
Do I need to dial 10 digits for a 331 number?
Yes, 10-digit dialing is required for all local calls in the 331 and 630 area code region. This means dialing the 3-digit area code followed by the seven-digit telephone number.
What area code overlays 331?
The 331 area code overlays the 630 area code. Both area codes serve the exact same geographic region in the western suburbs of Chicago.
When was the 331 area code created?
The 331 area code was introduced as an overlay for the 630 area code. It went into service on October 7, 2007 to provide additional telephone numbers for the rapidly growing western suburban region.
Can I get a business phone number with the 331 area code?
Yes, many VoIP and telecom service providers offer 331 numbers for businesses. A 331 number helps businesses establish a trustworthy local presence in Chicago's affluent western suburbs.
Is a 331 number good for local businesses?
Yes, a 331 number is ideal for businesses targeting customers in Aurora, Naperville, Wheaton, Downers Grove, Elmhurst, Bolingbrook, and surrounding cities. It establishes local identity and increases answer rates.
Can I keep my 331 number if I move?
In most cases, yes. Mobile and VoIP numbers can typically be retained even if you relocate to another city or state, depending on your phone carrier and service plan.
Why am I getting calls from a 331 number?
A call from a 331 number may come from a resident, business, school, hospital network (such as Edward-Elmhurst Health), or government agency in the western Chicago suburbs. If you do not recognize the number, let it go to voicemail and verify the caller before calling back.
What should I do if I get spam calls from a 331 number?
If you receive spam calls from a 331 number, never provide personal or financial details. Block the number, report it to your carrier and the FTC at ReportFraud.ftc.gov, and consider setting up an AI receptionist to screen unwanted automated robocalls.
Sources
Related posts
Insights
380 Area Code: Quick Overview, Location, Map, Scams
The 380 area code serves the Columbus metropolitan area in central Ohio, USA, covering Columbus, Worthington, and more. Explore time zones, uses, & scams.
12 min read
Insights
440 Area Code: Quick Overview, Location, Map, Uses, & Scams
The 440 area code serves northeastern Ohio, USA, covering Cleveland suburbs, Lorain, Elyria, Mentor, and more with Eastern Time details, cities list, dialing rules, and full location info.
12 min read
Insights
405 Area Code: Quick Overview, Location, Map, Uses, & Scams
The 405 area code serves central Oklahoma, USA, covering Oklahoma City, Norman, Edmond, and more with Central Time details, and full location info.
12 min read