| District of Illinois area codes: 815 | |
|---|---|
| Country | United States |
| State/Province | Illinois |
| Major City | Chicago |
| Time Zone | Central |
| Prefix | County | Primary City | Company |
|---|---|---|---|
| 815-322 | Mchenry | Mchenry, IL | Level 3 Communications |
| 815-323 | Boone | Belvidere, IL | Level 3 Communications |
| 815-324 | La Salle | Ottawa, IL | Mediacom Telephony Of Illinois |
| 815-325 | Will | Joliet, IL | New Cingular Wireless PCS |
| 815-326 | La Salle | Oglesby, IL | New Cingular Wireless PCS |
| 815-327 | Will | Plainfield, IL | Broadwing Communications |
| 815-328 | Will | Lockport, IL | Broadwing Communications |
| 815-329 | Winnebago | Rockford, IL | Sbc Internet Services |
| 815-330 | Iroquois | Crescent City, IL | First Communications LLC |
| 815-331 | Mchenry | Mchenry, IL | Broadwing Communications |
| 815-332 | Winnebago | Cherry Valley, IL | Frontier North |
| 815-333 | Mchenry | Woodstock, IL | Broadwing Communications |
| 815-334 | Mchenry | Woodstock, IL | Ameritech Illinois |
| 815-335 | Winnebago | Winnebago, IL | Frontier North |
| 815-336 | Carroll | Milledgeville, IL | Citizens Telecom Co Illinois-frontier Citizens -il |
| 815-337 | Mchenry | Woodstock, IL | Ameritech Illinois |
| 815-338 | Mchenry | Woodstock, IL | Ameritech Illinois |
| 815-339 | Putnam | Granville, IL | Frontier North |
| 815-340 | Bureau | La Moille, IL | Comcast Phone Of Illinois DBA Comco |
| 815-341 | Will | Joliet, IL | New Cingular Wireless PCS |
There are 785 prefixes, In the code area, the major cities are Belvidere, IL;Crystal Lake, IL;De Kalb, IL;Freepor. The major city is Chicago.