310
Area code
CA
United States geographic area code, in service since 1991-11-02. Part of the 310/424 overlay complex — ten-digit dialing required.
Split from 213. Created 562. Created 424. Overlaid with 424 .
Statusin service
Introduced1991-11-02
JurisdictionCA, US
Local dialing1+10D
Time zonePacific
Last verified2026-09-17
Utilization 101% of assignable prefixes assigned
As of 09/17/2026
0%800 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 2001-02-28, most recent 2025-11-20.
Area codes serve rate-center geographies, not municipal boundaries. “CA” identifies the jurisdiction, not an exact city footprint.
Top assignees
Ranked by assigned prefixes · initial assignee ≠ current carrier| Rank | Assignee | Prefixes | Share |
|---|---|---|---|
| 01 | PACIFIC BELL | 250 | 31.3% |
| 02 | FRONTIER CALIFORNIA INC. | 137 | 17.1% |
| 03 | NEW CINGULAR WIRELESS PCS, LLC | 68 | 8.5% |
| 04 | T-MOBILE USA, INC. | 58 | 7.2% |
| 05 | CELLCO PARTNERSHIP DBA VERIZON WIRELESS - CA | 56 | 7% |
| 06 | MCIMETRO ACCESS TRANSMISSION SERVICES LLC - CA | 27 | 3.4% |
| 07 | U.S. TELEPACIFIC CORP. DBA TPX COMMUNICATIONS | 24 | 3% |
| 08 | SPOK, INC. | 24 | 3% |
| 09 | LEVEL 3 COMMUNICATIONS, LLC - CA | 22 | 2.8% |
| 10 | TELEPORT COMMUNICATIONS AMERICA, LLC - CA | 20 | 2.5% |
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- 2005-09-07 NPA 424 to Overlay NPA 310 (California)
- 1991-11-02Area code 310 entered service (created from 213).
Planning letters & orders
Primary documents| Date | Document | Subject |
|---|---|---|
| 2005-09-07 | PL-347 ↗ | NPA 424 to Overlay NPA 310 (California) |
| 2000-11-10 | PL-250 ↗ | 310 NPA Splits, Creating 424 NPA (California) |
| 1990-08-31 | IL-90-08-055 ↗ | NANP - SPLIT OF 213 (CALIFORNIA) NUMBERING PLAN AREA |