331 Area Code: Aurora, Naperville & Chicago's West Suburbs

The 331 area code is the overlay that shares Chicago's western suburbs with 630 — the DuPage County belt anchored by Aurora and Naperville. It covers the same affluent, fast-growing corridor west of the city, from Wheaton and Elmhurst out to Bolingbrook. Added in 2007, 331 keeps the region supplied with numbers as 630 fills up.

This guide covers where the 331 area code is, how it overlaid 630, the I-88 technology corridor that defines the region, the area by the numbers, its time zone, the "is it California or Mexico?" mix-up, and how to get a 331 number.

Where is the 331 area code?

The 331 area code location is the near and far western suburbs of Chicago — mostly DuPage County, plus portions of Cook, Kane, Kendall, and Will counties. It does not cover the city of Chicago itself (312, 773) or the northern suburbs (847).

Cities and communities inside the 331 / 630 footprint include:

How 331 overlaid 630

The western suburbs have grown through two code changes:

• 1996: 630 splits off. The western suburbs were originally part of 708. As the Chicago suburbs exploded, 630 was created to serve DuPage County and the western belt.
• 2007: the 331 overlay. Just over a decade later, 630 was running short of numbers. Rather than split the region again, regulators added the 331 overlay across the entire 630 area on October 7, 2007, and ten-digit dialing became mandatory.

Existing 630 numbers were untouched; 331 simply adds a fresh supply of numbers for the same territory.

How 630 and 331 work together

Because 331 overlays 630, the two codes cover identical ground — neither signals a different town or carrier:

• Your existing 630 number does not change.
• Newer lines may be assigned 331, while established ones keep 630.
• Ten-digit dialing is required on every call, even local ones.
• Calls between 630 and 331 are local, with no long-distance charge between them.

The 331 code has no territory of its own — it exists only as the overlay on the 630 area code.

The I-88 Technology and Research Corridor

The 331/630 region is one of the strongest suburban economies in the Midwest, strung along Interstate 88 — the "Illinois Technology and Research Corridor":

• Science and research — Fermilab, the national particle-physics laboratory, sits in Batavia, anchoring a deep research presence.
• Corporate headquarters — Naperville and Downers Grove host major corporate campuses across energy, telecom, and consumer goods.
• Healthcare and education — large hospital systems plus colleges like North Central College and the Aurora University system.
• Retail and logistics — affluent suburbs and strong highway access support major retail and distribution operations.

For tech firms, professional services, healthcare groups, and contractors, a local 331 or 630 number reads as genuinely part of this prosperous western-suburb market.

The 331 area code by the numbers

A few figures capture why the region needed a second code:

• Aurora is the second-most-populous city in Illinois, behind only Chicago.
• DuPage County is among the most affluent and densely populated counties in the state.
• The overlay added in 2007 roughly doubled the available number supply for the same area.
• The region runs on Central Time and uses mandatory ten-digit dialing.

331 time zone and dialing

The 331 area code time zone is Central Time (CT) — CST (UTC−6) in winter and CDT (UTC−5) in summer, the same clock as Chicago. Because 331 shares an overlay with 630, ten-digit dialing is required on every call, and the same applies to cell-phone calls and text messages: dial 1 331 XXX XXXX from elsewhere in the US, or +1 331 XXX XXXX internationally and from mobiles.

Is 331 a California or Mexico number?

People sometimes search whether 331 belongs to California or Mexico — so to be clear: 331 is an Illinois area code, not a California or Mexican one. It serves Chicago's western suburbs. California uses codes like 213 and 415; Mexico uses an entirely different country code (+52). If a "+1 331" caller claims to be elsewhere, treat it as a sign of spoofing.

Putting a 331 number to work for your business

A 331 number puts a western-suburb Chicago address on your caller ID without a local lease. For tech firms, professional services, healthcare groups, and contractors selling into DuPage County, a local code reads as familiar and lifts answer rates over an unknown out-of-state or toll-free line.

You do not need a Naperville office to claim one. A cloud phone provider can assign you a 331 number that rings and texts on your existing devices and routes calls anywhere. With a provider like My Country Mobile, you can pick a 331 line, set up routing, an auto-attendant, and voicemail, and be live in minutes.

Staying safe from 331 spam calls

Scammers spoof 331 numbers — faking a western-suburb prefix, sometimes matching the first digits of your own number — so a call or text looks local. A 331 on your screen does not prove the contact is really in Illinois.

A few habits keep you safe:

• Let unknown 331 or 630 calls go to voicemail, and don't tap links in unexpected texts.
• Never share one-time codes, card numbers, or login details with an inbound caller or texter.
• Be skeptical of "utility shutoff," "package delivery," or "your account is compromised" messages that pressure you to act fast.
• Use carrier spam filtering. The FCC's STIR/SHAKEN call authentication framework is why many phones now label these as "Spam Risk."

Key takeaways

• 331 overlays 630 in Chicago's western suburbs — Aurora, Naperville, and DuPage County.
• It was added in 2007, a decade after 630 split from 708 in 1996.
• The region runs on the I-88 technology corridor, Fermilab, and corporate campuses, on Central Time.
• It is in Illinois, not California or Mexico.
• You can get a 331 number from the cloud without an Illinois address.

Frequently asked questions