Area code 312

As a result of Illinois' dense population, particularly in the north, the state was split between four NPAs, given that each NPA could only accommodate approximately 500 central offices.

The major exceptions were portions of Grundy, Kane, Kankakee, McHenry, and Will counties, which were located in the numbering plan area of the rest of northern Illinois, 815.

The 1989 split was intended as a long-term solution to relieve exchanges in one of the largest toll-free calling zones in the nation.

However, within only five years, 312 was on the verge of exhaustion once again due to the proliferation of cell phones, pagers and fax machines in the 1990s.

The Illinois side of the Chicago area–312/773/872, 708/464, 847/224, 630/331 and portions of 815/779–is one of the largest local calling areas in the United States; with few exceptions, no long-distance charges are applied from one portion of the metro area to the other.