Area code
312
312 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 312
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.
Digits with or without punctuation work. Numbers outside the US are read with the +country code.
Area code 312 at a glance
NANPA lists 784 central office codes (prefixes) in use in area code 312, held by 35 holders across 8 rate centers.
| Fact | Value |
|---|---|
| In service since | 1947-01-01 |
| Time zone | Central (C) |
| Assigned overlays | 872 (2009-11-07) |
| Prefixes in use | 784 |
| Pooled prefixes | 413 (53%) |
| Rate centers | 8 |
| Companies holding prefixes | 35 |
| Assignment dates on file | 2001-02-14 to 2026-01-29 (199 prefixes) |
| ZIP codes in its places | 7 |
Rate centers in area code 312
The largest rate center is CHICGOZN01, with 746 prefixes (95%).
| Rate center | Prefixes | Share |
|---|---|---|
| CHICGOZN01 | 746 | 95% |
| WHEELING | 12 | 2% |
| HINSDALE | 11 | 1% |
| LA GRANGE | 8 | 1% |
| NORTHBROOK | 3 | <1% |
| ROSELLE | 2 | <1% |
| CHICGOZN04 | 1 | <1% |
| CHICGOZN07 | 1 | <1% |
Who holds the prefixes in 312
Illinois Bell Tel holds the most prefixes, 334 (43%). 53% of the prefixes are pooled, which means their thousands-blocks can belong to different companies.
| Holder | Prefixes | Share |
|---|---|---|
| Illinois Bell Tel | 334 | 43% |
| New Cingular Wireless PCS | 82 | 10% |
| T-Mobile USA | 64 | 8% |
| Verizon Wireless | 54 | 7% |
| Mcimetro Access Transmission Services | 40 | 5% |
| Level 3 Communications | 33 | 4% |
| 29 other carriers | 177 | 23% |
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 312
- 1947-01-01 Area code 312 went into service.
- 1989-11-11 Area code 708 went into service and names 312 as its parent.
- 1996-10-12 Area code 773 went into service and names 312 as its parent.
- 2009-11-07 Area code 872 went into service as an overlay in the same area as 312.
Area codes nearest to 312
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 312
7 ZIP codes fall in places that have a rate center in this area code.
| Place | ZIP codes |
|---|---|
| Hinsdale, IL | 2 |
| Northbrook, IL | 2 |
| La Grange, IL | 1 |
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 312 numbers
5,393 FCC complaints are on file for 3,016 numbers in 312, most often filed as "Unwanted Calls". These are unverified consumer reports.
| Year of latest complaint | Numbers |
|---|---|
| 2023 | 174 |
| 2024 | 206 |
| 2025 | 108 |
Other area codes in Illinois
Illinois is one state with 17 area codes in the NANPA table. 312 is one of them.
Next to 312 in the numbering table
These are the codes just before and after 312 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 312 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 312 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 312
Where is area code 312?
NANPA assigns area code 312 to Illinois, United States.
In which state does area code 312 sit?
Illinois. 312 is one of 17 area codes the NANPA table assigns to the state.
Which places does area code 312 cover?
The places with the most ZIP codes among the rate centers in 312 are Hinsdale, IL, Northbrook, IL and La Grange, IL. The rate center list on this page shows all of them.
Who owns area code 312?
NANPA assigns area codes. Within 312, Illinois Bell Tel holds the most prefixes, 334 prefixes or 43%.
Which time zone does 312 use?
The NANPA NPA report lists the time zone for 312 as Central (C).
What year did 312 enter service?
The NANPA NPA report gives an in-service date of 1947-01-01, so 312 has been in use since 1947.
Does area code 312 have an overlay?
Yes. 872 (in service 2009-11-07) shares the same area with 312, so numbers from all of them are local to each other.
Which area codes are nearest to 312?
The area codes closest to 312, measured between the centers of the ZIP code places they serve, are 847 (about 11 miles), 224 (about 12 miles), 630 (about 15 miles), 708 (about 17 miles) and 331 (about 18 miles).
How many prefixes does area code 312 have?
NANPA's report dated 2026-10-17 lists 784 prefixes in service in 312, spread over 8 rate centers and 35 holders.
Are there complaints about 312 numbers?
The FCC data on file has 5,393 complaints for 3,016 numbers in 312. 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.