| 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 |
|---|---|---|---|
| 312-412 | Cook | Wheeling, IL | New Cingular Wireless PCS |
| 312-413 | Cook | Chicago, IL | Ameritech Illinois |
| 312-415 | Cook | Chicago, IL | Cellco Partnership DBA Verizon Wireless |
| 312-416 | Cook | Chicago, IL | Mci Worldcom Communications |
| 312-417 | Cook | Chicago, IL | Ameritech Illinois |
| 312-418 | Cook | Chicago, IL | Usa Mobility Wireless |
| 312-419 | Cook | Chicago, IL | Ameritech Illinois |
| 312-420 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-421 | Cook | Chicago, IL | Ameritech Illinois |
| 312-422 | Cook | Chicago, IL | Ameritech Illinois |
| 312-423 | Cook | Chicago, IL | Broadwing Communications |
| 312-424 | Cook | Chicago, IL | Ameritech Illinois |
| 312-425 | Cook | Chicago, IL | Ameritech Illinois |
| 312-426 | Cook | Chicago, IL | Usa Mobility Wireless |
| 312-427 | Cook | Chicago, IL | Ameritech Illinois |
| 312-428 | Cook | Chicago, IL | Level 3 Communications |
| 312-429 | Cook | Chicago, IL | Global Crossing Local Services-il |
| 312-430 | Cook | Chicago, IL | Ameritech Illinois |
| 312-431 | Cook | Chicago, IL | Ameritech Illinois |
| 312-432 | Cook | Chicago, IL | Ameritech Illinois |
There are 701 prefixes, In the code area, the major cities are Chicago, IL. The major city is Chicago.