312
Area code
IL
United States geographic area code, in service since 1947-01-01. Part of the 312/872 overlay complex — ten-digit dialing required.
Statusin service
Introduced1947-01-01
JurisdictionIL, US
Local dialing1+10D
Time zoneCentral
Last verified2026-09-17
Utilization 100.8% of assignable prefixes assigned
As of 09/17/2026
0%798 of ~792 assignable NXX values (200–999 excl. N11)100%
Prefix-level assignment does not equal number-level exhaustion; thousands-block pooling extends capacity. First assignment on record 2001-02-14, most recent 2026-01-29.
Area codes serve rate-center geographies, not municipal boundaries. “IL” identifies the jurisdiction, not an exact city footprint.
Top assignees
Ranked by assigned prefixes · initial assignee ≠ current carrier| Rank | Assignee | Prefixes | Share |
|---|---|---|---|
| 01 | ILLINOIS BELL TEL CO | 334 | 41.9% |
| 02 | NEW CINGULAR WIRELESS PCS, LLC - IL | 82 | 10.3% |
| 03 | T-MOBILE USA, INC. | 64 | 8% |
| 04 | CELLCO PARTNERSHIP DBA VERIZON WIRELESS - IL | 54 | 6.8% |
| 05 | SPOK, INC. | 33 | 4.1% |
| 06 | MCIMETRO ACCESS TRANSMISSION SERVICES LLC | 33 | 4.1% |
| 07 | LEVEL 3 COMMUNICATIONS, LLC - IL | 33 | 4.1% |
| 08 | TELEPORT COMMUNICATIONS AMERICA, LLC - IL | 22 | 2.8% |
| 09 | ONVOY, LLC | 20 | 2.5% |
| 10 | AMERICAN MESSAGING SERVICES, LLC | 14 | 1.8% |
Important: these are initial assignees from NANPA CO-code records — the carrier a prefix was originally assigned to. Because of number portability, the current serving carrier of any individual number may differ.
Change history
Recorded events for this NPA- 2009-08-10 NPA 872 to Overlay NPA 312 and 773 (Illinois)
- 1947-01-01Area code 312 entered service.
Planning letters & orders
Primary documents| Date | Document | Subject |
|---|---|---|
| 2009-08-10 | PL-392 ↗ | NPA 872 to Overlay NPA 312 and 773 (Illinois) |
| 2009-06-15 | PL-390 ↗ | NPA 872 to Overlay NPA 312 and 773 (Illinois) |
| 1999-10-24 | PL-195 ↗ | NPA 872 to Overlay NPAs 312 and 773 (Illinois)NPA 464 to Overlay NPA 708NPA 331 to Overlay NPA 630 |