312 is the North American telephone area code for the state of Illinois, including the cities of and Chicago.
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| Prefix | Primary City | Company | County | Usage |
|---|---|---|---|---|
| 312-201-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-202-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-203-#### | Chicago, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-204-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-205-#### | Chicago, IL | Allegiance Telecom | Cook | Landline |
| 312-206-#### | Chicago, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-207-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-208-#### | Chicago, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-209-#### | Northbrook, IL | Cellco Partnership DBA Verizon Wireless | Cook | Cell Number |
| 312-210-#### | Chicago, IL | Cook | Landline | |
| 312-212-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-213-#### | Chicago, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-214-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-215-#### | Wheeling, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-216-#### | Cook | Cell Number | ||
| 312-217-#### | Chicago, IL | United States Cellular Corp. - Illinois | Cook | Cell Number |
| 312-218-#### | Chicago, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-220-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-221-#### | Sprint Spectrum L.p. | Cook | Cell Number | |
| 312-222-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-223-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-224-#### | Chicago, IL | Broadwing Communications | Cook | Landline |
| 312-225-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-226-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-227-#### | Chicago, IL | Ameritech Illinois | Unknown | |
| 312-228-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-229-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-230-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-231-#### | Roselle, IL | New Cingular Wireless PCS | Du Page | Cell Number |
| 312-232-#### | Chicago, IL | American Messaging (am) | Cook | Cell Number |
| 312-233-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-234-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-235-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-236-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-237-#### | Chicago, IL | Cellco Partnership DBA Verizon Wireless | Cook | Cell Number |
| 312-238-#### | Chicago, IL | Broadwing Communications | Cook | Landline |
| 312-239-#### | Chicago, IL | AT&T Local | Cook | Landline |
| 312-240-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-241-#### | Cook | Cell Number | ||
| 312-242-#### | Chicago, IL | Teleport Communications Group - Chicago | Cook | Landline |
| 312-243-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-244-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-245-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-246-#### | Wheeling, IL | New Cingular Wireless PCS | Cook | Cell Number |
| 312-247-#### | Chicago, IL | Globalcom-il | Cook | Landline |
| 312-249-#### | Chicago, IL | Usa Mobility Wireless | Cook | Cell Number |
| 312-250-#### | Chicago, IL | Usa Mobility Wireless | Cook | Cell Number |
| 312-251-#### | Chicago, IL | Ameritech Illinois | Cook | Landline |
| 312-252-#### | Chicago, IL | Allegiance Telecom | Cook | Landline |
| 312-253-#### | Chicago, IL | Allegiance Telecom | Cook | Landline |