231
Area code
Statusin service
Introduced1999-06-05
JurisdictionMI, US
Local dialing7D
Time zoneEastern
Last verified2026-09-17
Utilization 75.3% of assignable prefixes assigned
As of 09/17/2026
0%596 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 2001-01-09, most recent 2026-03-19.
Area codes serve rate-center geographies, not municipal boundaries. “MI” identifies the jurisdiction, not an exact city footprint.
Top assignees
Ranked by assigned prefixes · initial assignee ≠ current carrier| Rank | Assignee | Prefixes | Share |
|---|---|---|---|
| 01 | NEW CINGULAR WIRELESS PCS, LLC - IL | 72 | 12.1% |
| 02 | MICHIGAN BELL TEL CO | 72 | 12.1% |
| 03 | FRONTIER NORTH, INC. - MI | 52 | 8.7% |
| 04 | ONVOY, LLCV - MI | 49 | 8.2% |
| 05 | TELNET WORLDWIDE, INC. | 43 | 7.2% |
| 06 | CELLCO PARTNERSHIP DBA VERIZON WIRELESS - MI | 42 | 7% |
| 07 | 123.NET, LLC | 34 | 5.7% |
| 08 | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC | 28 | 4.7% |
| 09 | CHARTER IP ENABLED SERVICES, LLC | 20 | 3.4% |
| 10 | LEVEL 3 COMMUNICATIONS, LLC - MI | 19 | 3.2% |
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- 1999-06-05Area code 231 entered service (created from 616).
Planning letters & orders
Primary documents| Date | Document | Subject |
|---|---|---|
| 1999-03-11 | PL-167 ↗ | Geographic Split of 616 (Michigan) Numbering Plan Area (NPA) |