| 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 |
|---|---|---|---|
| 312-243 | Cook | Chicago, IL | Ameritech Illinois |
| 312-244 | Cook | Chicago, IL | Ameritech Illinois |
| 312-245 | Cook | Chicago, IL | Ameritech Illinois |
| 312-246 | Cook | Wheeling, IL | New Cingular Wireless PCS |
| 312-247 | Cook | Chicago, IL | First Communications LLC |
| 312-248 | Metro PCS | ||
| 312-249 | Cook | Chicago, IL | Usa Mobility Wireless |
| 312-250 | Cook | Chicago, IL | Usa Mobility Wireless |
| 312-251 | Cook | Chicago, IL | Ameritech Illinois |
| 312-252 | Cook | Chicago, IL | Allegiance Telecom |
| 312-253 | Cook | Chicago, IL | Allegiance Telecom |
| 312-254 | Cook | Chicago, IL | Allegiance Telecom |
| 312-255 | Cook | Chicago, IL | Ameritech Illinois |
| 312-256 | Cook | Chicago, IL | Allegiance Telecom |
| 312-257 | Cook | Chicago, IL | Cellco Partnership DBA Verizon Wireless |
| 312-258 | Cook | Chicago, IL | Ameritech Illinois |
| 312-259 | Cook | Chicago, IL | New Cingular Wireless PCS |
| 312-260 | Cook | Chicago, IL | Mcimetro Access Transmission Services LLC |
| 312-261 | Cook | Chicago, IL | Broadwing Communications |
| 312-262 | Cook | Chicago, IL | Allegiance Telecom |
There are 701 prefixes, In the code area, the major cities are Chicago, IL. The major city is Chicago.