| District of Illinois area codes: 312 | |
|---|---|
| Country | United States |
| State/Province | Illinois |
| Major City | Chicago |
| Time Zone | Central (UTC-06:00) |
| Prefix | County | Primary City | Company |
|---|---|---|---|
| 312-201 | Cook | Chicago, IL | Ameritech Illinois |
| 312-202 | Cook | Chicago, IL | Ameritech Illinois |
| 312-203 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-204 | Cook | Chicago, IL | Ameritech Illinois |
| 312-205 | Cook | Chicago, IL | Allegiance Telecom |
| 312-206 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-207 | Cook | Chicago, IL | Ameritech Illinois |
| 312-208 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-209 | Cook | Northbrook, IL | Cellco Partnership DBA Verizon Wireless |
| 312-212 | Cook | Chicago, IL | Ameritech Illinois |
| 312-213 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-214 | Cook | Chicago, IL | Ameritech Illinois |
| 312-215 | Cook | Wheeling, IL | New Cingular Wireless PCS |
| 312-216 | Cook | Chicago, IL | First Communications LLC |
| 312-217 | Cook | Chicago, IL | United States Cellular Corp. - Illinois |
| 312-218 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-219 | Cook | Chicago, IL | Bandwidth.com Clec |
| 312-220 | Cook | Chicago, IL | Ameritech Illinois |
| 312-221 | Cook | Chicago, IL | Sprint Spectrum L.p. |
| 312-222 | Cook | Chicago, IL | Ameritech Illinois |
There are 701 prefixes, In the code area, the major cities are Chicago, IL. The major city is Chicago.