---
title: "Area code 312: Illinois, United States"
url: "https://lookvalid.com/area-code/312.md"
canonical_url: "https://lookvalid.com/area-code/312"
last_updated: "2026-10-23"
type: "reference"
summary: "Area code 312 serves Illinois, United States. Number formats, numbering rules, a live phone validator and the other codes in Illinois."
---

# Area code 312: Illinois, United States

Area code 312 serves Illinois, United States. Number formats, numbering rules, a live phone validator and the other codes in Illinois.

NANPA lists 784 central office codes (prefixes) in use in area code 312, held by 35 holders across 8 rate centers.
The largest rate center is CHICGOZN01, with 746 prefixes (95%).
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.
The FCC unwanted-call data has 5,393 complaints on file for 3,016 numbers in 312, most often filed as "Unwanted Calls". These are consumer reports, not findings.

## Facts

- 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

| 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% |

## Carriers

| 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.

## History

- 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.

## Nearest area codes

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).

## ZIP codes

7 ZIP codes fall in places that have a rate center in this area code. The places with the most are Hinsdale, IL (2 ZIP codes), Northbrook, IL (2 ZIP codes) and La Grange, IL (1 ZIP code). 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

5,393 complaints for 3,016 numbers. Numbers whose latest complaint falls in each year:

- 2023: 174
- 2024: 206
- 2025: 108

## Questions

### 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.

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).