The 331 area code is an overlay serving the western suburbs of Chicago, layered over the 630 code across DuPage County and beyond. Introduced in 2007 to supply new phone numbers, 331 covers the exact same territory as 630. If a 331 number has appeared on your caller ID, this guide explains where it comes from, how to dial it, and how to stay safe from spoofed calls.
Quick Summary: Area Code 331 at a Glance
| Detail | Information |
|---|---|
| Area Code | 331 |
| State | Illinois (IL) |
| Major Cities | Aurora, Naperville, Wheaton, Downers Grove |
| Time Zone | Central Time (CT) |
| Country Code | +1 (USA) |
| Overlay Codes | 630 (co-located) |
Geographic Coverage & Major Cities
Area code 331 covers Chicago’s western suburbs, centered on DuPage County and reaching into parts of Kane, Will, and Cook counties. Because it is an overlay, it shares the identical footprint of the 630 area code rather than covering a separate zone.
- Aurora – the second-largest city in Illinois
- Naperville – a large, highly rated suburb west of the city
- Wheaton & Downers Grove – established DuPage County communities
- Elmhurst, Lombard & Glen Ellyn – inner western suburbs
As the newer code over the western suburbs, 331 numbers are typically assigned to lines set up after the original 630 numbers grew scarce.
A Brief History of Area Code 331
Area code 331 was activated in 2007 as an all-services overlay on the 630 region. The 630 code itself split from 708 in 1996. An overlay adds a second area code across the same geographic area, so existing 630 customers kept their numbers while new lines could be assigned either 630 or 331, under mandatory 10-digit dialing.
Time Zone & Best Time to Call
Numbers in the 331 area code use Central Time (CT). The region observes Daylight Saving Time, shifting from CST (UTC-6) to CDT (UTC-5) between March and November each year.
How to Dial Phone Numbers in Area Code 331
Because 331 shares its region under an overlay, the area requires 10-digit dialing on every call.
Calling within the United States or Canada
- 10-digit dialing: Include the area code – 331-XXX-XXXX.
- 11-digit dialing: You can also dial 1-331-XXX-XXXX; the leading 1 is the NANP country code.
Calling from outside the United States
- Format: [exit code] + 1 + 331 + XXX + XXXX
- From the UK, Europe, or most of Asia/Africa: 00 1 331 XXX XXXX
- From any mobile phone worldwide: +1 331 XXX XXXX
Is Area Code 331 a Scam? Safety & Protection Tips
Area code 331 is a legitimate Illinois area code – a call from it is not fraudulent by default. However, scammers frequently spoof local codes like 331, faking the caller ID so their number appears to come from a trusted local line. Neighbor spoofing, which mimics the first six digits of your own number, is especially common.
Common scam tactics linked to this region
| Scam type | What to watch for |
|---|---|
| Government impersonation | Callers posing as the IRS or SSA threatening arrest or fines |
| Bank & card fraud alerts | Claims of suspicious activity seeking your account details |
| Utility shut-off threats | Fake ComEd or Nicor Gas disconnection demands |
| Package delivery scams | Fake texts or calls about a failed delivery with a bad link |
How to report, block, and look up 331 callers
- Never share personal or financial details with an unexpected caller, however urgent they sound.
- Block the number using your phone’s built-in call-blocking feature or a reputable call-screening app.
- Look up the number with a trusted reverse phone lookup before returning any call.
- Report it to the FTC at reportfraud.ftc.gov and register at DoNotCall.gov.
Nearby & Related Area Codes in Illinois
Area code 331 shares and borders several other Illinois codes:
- 630 – the original western-suburbs code that 331 overlays (see our 630 area code guide)
- 312, 773 & 872 – the City of Chicago to the east (see our 312 area code guide)
- 847 & 224 – the northern suburbs (see our 847 area code guide)
- 815 & 779 – northern Illinois beyond the metro (see our 815 area code guide)
Explore more codes across the state on our Illinois area codes hub, or browse every prefix in our US area codes directory.
↑ See the complete list of All US Area Codes by State – every area code across all 50 states in one directory.
Explore Neighboring States
Researching area codes just across the state line? Start with these codes in the states that border Illinois:
- Wisconsin – area code 414
- Iowa – area code 515
- Missouri – area code 314
- Kentucky – area code 502
- Indiana – area code 317
Frequently Asked Questions About Area Code 331
Where is the 331 area code located?
Area code 331 covers the western suburbs of Chicago in DuPage County, including Aurora, Naperville, and Wheaton. It operates on Central Time as an overlay of area code 630.
What is the difference between 331 and 630?
There is no geographic difference. 331 is an overlay covering the exact same western-suburb territory as 630; it was added in 2007 simply to supply more phone numbers.
Is 331 a landline or cell phone area code?
Both. Area code 331 serves landlines, mobile phones, and VoIP numbers throughout the western suburbs.
Should I answer a call from an unknown 331 number?
If you are not expecting the call, let it go to voicemail. Legitimate callers leave a message, while spoofed or robocall numbers usually will not. Verify any unfamiliar number with a reverse lookup before calling back.
Related 331 Guides
- Is a 331 call safe? Read Is area code 331 a scam? Spam & safety guide.
- Local coverage: see the Aurora area code guide.
- Worried about spam or robocalls? Browse our Scam Alerts directory.