| 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 |
|---|---|---|---|
| 312-390 | Cook | Chicago, IL | American Messaging (am) |
| 312-391 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-393 | Cook | Chicago, IL | American Messaging (am) |
| 312-394 | Cook | Chicago, IL | Ameritech Illinois |
| 312-395 | Cook | Chicago, IL | AT&T Local |
| 312-396 | Cook | Chicago, IL | Ameritech Illinois |
| 312-397 | Cook | Chicago, IL | Ameritech Illinois |
| 312-398 | Cook | Chicago, IL | Usa Mobility Wireless |
| 312-399 | Cook | Chicago, IL | Sprint Spectrum L.p. |
| 312-400 | Cook | Chicago, IL | American Messaging (am) |
| 312-401 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-402 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-403 | Cook | La Grange Park, IL | Cellco Partnership DBA Verizon Wireless |
| 312-404 | Cook | Chicago, IL | T-mobile Usa |
| 312-405 | Cook | Chicago, IL | Cellco Partnership DBA Verizon Wireless |
| 312-406 | Cook | Wheeling, IL | New Cingular Wireless PCS |
| 312-407 | Cook | Chicago, IL | Ameritech Illinois |
| 312-408 | Cook | Chicago, IL | Ameritech Illinois |
| 312-409 | Cook | Chicago, IL | Ameritech Illinois |
| 312-410 | Cook | Chicago, IL | Ameritech Illinois |
There are 701 prefixes, In the code area, the major cities are Chicago, IL. The major city is Chicago.