Significant events for area code 312
| Date |
Details |
| 1947 | One of the four original area codes for Illinois. It covered the Chicago metro area. |
| 1989 | Area code 312 was split with 708 covering the Chicago suburbs. |
| 1996 | Area code 312 was split with 773 covering all of the area outside of downtown Chicago. |
| 2009 | Area code 872 was overlaid on 312 and 773 . |