Introduction
The 331 area code is not a new region of its own — it is a second layer of phone numbers spread across the exact same territory as Chicago's western-suburbs code, 630. It reaches Aurora and Naperville at its core, Bolingbrook and Wheaton nearby, and stretches out to Yorkville in Kendall County and a slice of Cook County around Schaumburg and Streamwood.
331 went into service on October 7, 2007 as an all-services overlay, added on top of 630 rather than carved out of it, so every existing number stayed exactly the same.
Most people looking up 331 fall into two groups: those assigned a 331 number who want to understand what it represents, and those who received a call from a 331 number and want to know whether it is a genuine western-suburbs line or a spoofed one.
This guide covers what 331 is, the full four-county footprint it shares with 630, how the overlay came about, its Central Time zone, the region's economy, common scam patterns, and how to activate a 331 virtual number for a business.
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Learn moreWhat Is the 331 Area Code?
The 331 area code is a telephone code covering Chicago's western suburbs, layered directly over the existing 630 code as an all-services overlay. It carries no separate geographic meaning of its own — a 331 number and a 630 number can belong to neighbors on the same Naperville street.
The Illinois Commerce Commission approved 331 once 630 neared exhaustion from suburban population growth and the rise of cell phones and pagers in the 2000s, and the code went live on October 7, 2007. New lines assigned anywhere in the region after that date could receive either 630 or 331, while every number already in service kept its original code unchanged.
Dialing Format and Time Zone
Because 331 overlays 630 across the same ground, the region requires ten-digit dialing for every local call, a rule that arrived with the overlay in 2007. Anyone unfamiliar with the standard format can review the US phone number format before dialing.
- Dialing format: (331) 555-0148
- International: +1 331-555-0148
- Overlay of: 630, in service across the same territory since 1996
- Ten-digit dialing is mandatory throughout, including local calls between two western-suburbs numbers
Where Is 331? Cities & Coverage
The 331 footprint spans four counties west of Chicago — the bulk of DuPage County, southern Kane County around Aurora, northern Will County, and Kendall County — plus a pocket of northwest Cook County. It shares this exact ground with the 630 area code.
DuPage County and the Fox Valley
- Aurora — Illinois' second-largest city, straddling the Fox River across Kane, DuPage, Will and Kendall counties
- Naperville — one of the state's largest suburbs, anchoring DuPage County's east side
- Wheaton — the DuPage County seat, a historic western-suburbs hub
- Downers Grove & Bolingbrook — established suburban communities along the I-355/I-55 corridors
Beyond DuPage
- Yorkville — the Kendall County seat, anchoring the region's fast-growing southwest edge
- Joliet-area Will County communities — the overlay's reach into the region's south
- Schaumburg, Streamwood & Hanover Park — the pocket of northwest Cook County sharing the same code
Because 331 shares its footprint exactly with 630, the two are best understood together — a business comparing the older code to its overlay sees the identical territory either way.
331's History: An Overlay of 630
630 has served Chicago's western suburbs since August 3, 1996, when it split off from the old 708 area code in a three-way division that also created the northern-suburbs code, 847. For a decade, 630 alone covered the region's rapid growth.
Why an Overlay Instead of Another Split
By the mid-2000s, regulators projected that 630's remaining number supply would run out, driven by suburban population growth and the spread of cell phones and pagers. Rather than draw a new boundary through DuPage County and force some residents to change their code again, the Illinois Commerce Commission approved an overlay: a second code, 331, spread across the identical territory.
- August 3, 1996 — 630 splits from 708 to cover Chicago's western suburbs
- Late June 2007 — 630 reaches exhaust, with no unassigned prefixes left
- October 7, 2007 — 331 goes into service as a same-territory overlay, and ten-digit dialing becomes mandatory
That decision meant every existing 630 number stayed exactly the same, while new lines issued after 2007 could be assigned either code — a business calling itself "local Naperville" might legitimately carry a 331 number rather than the more familiar 630.
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331 Time Zone
The entire 331 overlay territory sits in the Central Time zone, alongside the rest of the Chicago metro area. There is no internal time-zone split to account for, unlike some larger states.
- Central Time (CT) — the whole 331/630 footprint, from Aurora to Yorkville to the Cook County pocket
- Same zone as downtown Chicago (312/773) and every neighboring suburban code
- No daylight-saving offset to manage internally — the region moves together
A business running a 331 line can schedule calls around a single Central Time clock without worrying about a split-zone customer base.
The Western Suburbs Economy and 331
A 331 number connects a business to one of the Chicago area's most productive suburban economies. Aurora carries a nickname earned in 1881, when it became one of the first cities in the United States to install a citywide electric street-lighting system — "The City of Lights" — a legacy that still anchors its civic identity today.
Corporate Campuses and Logistics
DuPage County hosts a dense concentration of corporate offices and research campuses along the I-88 corridor, while the I-55/I-80 junction near Bolingbrook and Joliet-area Will County supports a major logistics and distribution hub feeding the broader Chicago region.
Education and Community
Naperville and Wheaton anchor strong public-school systems and higher-education institutions that draw families and employers alike, while Yorkville's growth reflects the region's continued expansion into Kendall County.
Common 331 Phone Scams to Watch For
331 is a legitimate, officially assigned Illinois code, but like any area code its numbers can be spoofed. Caller-ID spoofing lets a fraudster display any number — including a 331 prefix — on a recipient's screen, so a call can appear to come from a local Aurora or Naperville business or agency when it does not.
- Unsolicited calls demanding immediate payment by gift card, wire transfer or cryptocurrency
- Callers posing as a utility, bank or county agency asking to "verify" personal or account details
- Fake warnings about an unpaid toll, tax bill or benefit problem that pressure an urgent callback
- Robocalls offering a prize or refund that require sharing card details to "process" it
The Illinois Commerce Commission's consumer resources cover how carriers assign and protect numbers like 331, and the safest habit is still to verify independently before acting on any unexpected call rather than trusting the name or number displayed on a screen.
How to Get a 331 Number
Modern cloud phone providers let a business activate a 331 number in minutes — no DuPage County office, no hardware, and no long-term contract. Search available 331 numbers, complete a short verification, and route the line to any device anywhere in the world.
A 331 number suits any business targeting the western suburbs — logistics and distribution operators near the I-55/I-80 junction, corporate and professional services along the I-88 corridor, and retailers or service providers reaching Aurora, Naperville, Bolingbrook and Yorkville alike. A company comparing 331 to the region's original 630 area code can pick either freely, since both cover identical ground.
MeraTalk provides 331 numbers as part of its cloud telephony platform — lines activate in minutes, route to any device, and pair with voicemail, call recording, an auto-attendant and CRM integration from one dashboard. A business that already runs a virtual phone number elsewhere can add a 331 line as a second local number in the same account. To get started, search available numbers at meratalk.com. For more, see Illinois Commerce Commission's consumer resources.
Conclusion
The 331 area code is the western suburbs' overlay — a second code layered over the same DuPage, Kane, Will and Kendall County territory the 630 area code has served since 1996, from Aurora and Naperville to Bolingbrook, Wheaton and Yorkville. Added in 2007 once 630 neared exhaustion, it remains a straightforward same-ground overlay serving one of the Chicago region's busiest suburban economies.
Activating a 331 virtual number takes minutes with MeraTalk. Go live with a genuine western-suburbs presence, route calls to any device, and add call recording, an auto-attendant and CRM integration from day one — no physical office in the region required. Explore the options at meratalk.com.
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