--- title: "Area code 331: Illinois, United States" url: "https://lookvalid.com/area-code/331.md" canonical_url: "https://lookvalid.com/area-code/331" last_updated: "2026-10-23" type: "reference" summary: "Area code 331 serves Illinois, United States. Number formats, numbering rules, a live phone validator and the other codes in Illinois." --- # Area code 331: Illinois, United States Area code 331 serves Illinois, United States. Number formats, numbering rules, a live phone validator and the other codes in Illinois. NANPA lists 222 central office codes (prefixes) in use in area code 331, held by 25 holders across 24 rate centers. 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. The FCC unwanted-call data has 860 complaints on file for 607 numbers in 331, most often filed as "Unwanted Calls". These are consumer reports, not findings. ## Facts - 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 | 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% | ## Carriers | 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. ## History - 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. ## Nearest area codes 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). ## ZIP codes 39 ZIP codes fall in places that have a rate center in this area code. The places with the most are Aurora, IL (10 ZIP codes), Naperville, IL (6 ZIP codes) and Glen Ellyn, IL (2 ZIP codes). A ZIP code and an area code are unrelated systems, so a ZIP code can sit in more than one area code and an area code spans many ZIP codes. ## Complaints on file 860 complaints for 607 numbers. Numbers whose latest complaint falls in each year: - 2023: 88 - 2024: 62 - 2025: 35 ## Questions ### 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. Sources: NANPA central office code report dated 2026-10-17; NANPA NPA report dated 2026-10-18; FCC unwanted-call complaints, data fetched 2026-10-17; ZIP code places from GeoNames (CC BY 4.0).