Area code
331
331 is a United States area code. Under the North American Numbering Plan it belongs to Illinois, which has 17 area codes in total.
Validate a number in 331
Paste a full number and the tool checks it against the numbering plan. It returns the standard formats, the line type the plan assigns and the area code region.
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Area code 331 at a glance
NANPA lists 222 central office codes (prefixes) in use in area code 331, held by 25 holders across 24 rate centers.
| Fact | Value |
|---|---|
| In service since | 2007-10-07 |
| Time zone | Central (C) |
| Assigned overlays | 630 (1996-08-03) |
| Prefixes in use | 222 |
| Pooled prefixes | 221 (100%) |
| Rate centers | 24 |
| Companies holding prefixes | 25 |
| Assignment dates on file | 2007-07-06 to 2026-05-18 (211 prefixes) |
| ZIP codes in its places | 39 |
Rate centers in area code 331
| Rate center | Prefixes | Share |
|---|---|---|
| AURORA | 45 | 20% |
| NAPERVILLE | 33 | 15% |
| DOWNERSGRV | 21 | 9% |
| ST CHARLES | 17 | 8% |
| LOMBARD | 13 | 6% |
| ELMHURST | 10 | 5% |
| LEMONT | 10 | 5% |
| BARTLETT | 8 | 4% |
| 16 other rate centers | 65 | 29% |
Who holds the prefixes in 331
T-Mobile USA holds the most prefixes, 42 (19%). 100% of the prefixes are pooled, which means their thousands-blocks can belong to different companies.
| Holder | Prefixes | Share |
|---|---|---|
| T-Mobile USA | 42 | 19% |
| Onvoy | 38 | 17% |
| Verizon Wireless | 33 | 15% |
| New Cingular Wireless PCS | 25 | 11% |
| Fractel | 18 | 8% |
| Bandwidth.com CLEC | 11 | 5% |
| 19 other carriers | 55 | 25% |
A prefix holder is the company a code was assigned to, not necessarily a number's current carrier, because numbers can be ported to another company. Find which carrier a number is on, or browse the prefix holders.
History of area code 331
- 1996-08-03 Area code 630, named as the parent of 331 in the NANPA report, was in service.
- 2007-10-07 Area code 331 went into service.
Area codes nearest to 331
Distance is measured between the centers of the ZIP code places each area code serves, so it is a rough guide.
Overlay codes in the same area
ZIP codes in area code 331
39 ZIP codes fall in places that have a rate center in this area code.
| Place | ZIP codes |
|---|---|
| Aurora, IL | 10 |
| Naperville, IL | 6 |
| Glen Ellyn, IL | 2 |
A ZIP code and an area code are unrelated systems. A ZIP code can sit in more than one area code, and an area code spans many ZIP codes.
Complaints on file for 331 numbers
860 FCC complaints are on file for 607 numbers in 331, most often filed as "Unwanted Calls". These are unverified consumer reports.
| Year of latest complaint | Numbers |
|---|---|
| 2023 | 88 |
| 2024 | 62 |
| 2025 | 35 |
Other area codes in Illinois
Illinois is one state with 17 area codes in the NANPA table. 331 is one of them.
Next to 331 in the numbering table
These are the codes just before and after 331 when all are sorted by number. They are neighbours on the list, not necessarily on the map.
What the area code does and does not tell you
A number that starts with 331 was first assigned in Illinois. That is a fact about where the number began, and it is a useful hint when you check a form entry, for example a billing address in Illinois next to a 331 phone number.
It does not show where the person is today. A number stays with its owner after a move or a change of provider (number portability), and mobile numbers travel with the handset. The area code also cannot tell you whether the line is active, who the carrier is, or whether the number is mobile or fixed: the plan shares those ranges. Read more about the area code, the numbering plan and line type.
Check numbers in bulk with the API
| Product | Credits | Docs |
|---|---|---|
| Phone Numbering-Plan Validation | 1 credit per valid number | API docs |
| Phone Intelligence (carrier and line status) | 1 credit per check (invalid numbers outside the US and Canada are free) | Product page |
Questions about area code 331
Where is area code 331?
NANPA assigns area code 331 to Illinois, United States.
In which state does area code 331 sit?
Illinois. 331 is one of 17 area codes the NANPA table assigns to the state.
Which places does area code 331 cover?
The places with the most ZIP codes among the rate centers in 331 are Aurora, IL, Naperville, IL and Glen Ellyn, IL. The rate center list on this page shows all of them.
Who owns area code 331?
NANPA assigns area codes. Within 331, T-Mobile USA holds the most prefixes, 42 prefixes or 19%.
Which time zone does 331 use?
The NANPA NPA report lists the time zone for 331 as Central (C).
What year did 331 enter service?
The NANPA NPA report gives an in-service date of 2007-10-07, so 331 has been in use since 2007.
Does area code 331 have an overlay?
Yes. 630 (in service 1996-08-03) shares the same area with 331, so numbers from all of them are local to each other.
Which area codes are nearest to 331?
The area codes closest to 331, measured between the centers of the ZIP code places they serve, are 312 (about 18 miles), 708 (about 23 miles), 847 (about 25 miles), 224 (about 26 miles) and 464 (about 27 miles).
How many prefixes does area code 331 have?
NANPA's report dated 2026-10-17 lists 222 prefixes in service in 331, spread over 24 rate centers and 25 holders.
Are there complaints about 331 numbers?
The FCC data on file has 860 complaints for 607 numbers in 331. They are unverified consumer reports. A number with no complaints on file is not proven safe.
Area code assignments: North American Numbering Plan Administrator NPA report, file dated 10/17/2026, retrieved 2026-10-17. Source file. New codes are assigned several times a year. Prefixes, rate centers, holders, time zone, in-service date and overlays: NANPA central office code report dated 2026-10-17 and NPA report dated 2026-10-18. Complaints: FCC consumer complaints open data, fetched 2026-10-17. ZIP code places: Postal code places and coordinates: GeoNames (geonames.org), licensed CC BY 4.0.