North American Numbers Source-cited · ownership disclosed
Area code
331
In service
United States · since 2007

West Chicago Suburbs (overlay of 630)

Area code 331 is an all-services overlay of 630, sharing the exact same west-suburban DuPage-centered geography. It took effect October 7, 2007 with mandatory ten-digit dialing.

The essentials Numbering data · NumberResearch · as of 2026-09-18
Place
West Chicago Suburbs (overlay of 630)
Type
geographic
Status
in service
In service since
2007
Projected exhaust
2080 Q1

Status, dates and counts are read from NumberResearch (NANPA / CNAC / FCC records). Assignment records show initial assignees, not the carrier serving a number today.

A · Where it is

Where 331 is

Same as 630 - majority of DuPage County, parts of northwest Cook County, southern Kane County, northern Will County, and Kendall County.

Communities served wholly or partly by 331:

AuroraNapervilleWheatonDowners GroveBolingbrookElmhurstCarol StreamAddisonOak BrookLombard

Same footprint as 630 (65+ municipalities); numbers assigned across the whole 630 territory, not a sub-region.

B · Relationships

How it relates to other codes

The codes 331 was split from, spawned, or shares its area with. A split divides the map and changes some numbers; an overlay shares the same ground and changes none — what's the difference?

Overlay · 2007 630 West Chicago Suburbs (DuPage County) 331 overlays 630 over identical geography; effective Oct 7, 2007.
What's changing. Overlay of 630 (not a split); both codes cover identical territory. Ten-digit dialing required.
C · History

How 331 got here

1999-08-16
331 assigned as the overlay code for 630.
Source: Illinois Commerce Commission chronology
2007-10-07
331 overlay of 630 took effect; mandatory ten-digit dialing.
Source: Wikipedia - Area codes 630 and 331
D · Capacity

How full it is

Prefixes assigned
239
Prefixes available
561
Thousands-blocks assigned
2,099

Prefix-assignment counts describe allocation to carriers, not how many individual numbers are in use. Full technical records: NumberResearch ↗.

Compare 331 with another code →
Sources
Wikipedia ↗
Area codes 630 and 331
Illinois Commerce Commission ↗
Chronological History of Area Code Relief in Illinois
NumberResearch.org ↗
Live status, dates, assignment counts and exhaust forecast — data as of 2026-09-18 (CC BY 4.0).

Place, relationships and history above are compiled by North American Numbers from the sources listed and last checked 2026-09-18. A check date is not an expert review — see how this is made. Flag an error.