Phone numbers in Diamond Bar, California
Diamond Bar has 32 phone prefixes in area codes 840, 909.
Biggest holders: Frontier, Onvoy, Bandwidth. This is the local calling area (rate center) where those prefixes were first assigned.
- Mobile carrier 6%
- Local phone company 41%
- Internet / business phone carrier 53%
Every prefix in Diamond Bar
Search the first six digits of a number in this list. If it's here, the number started in Diamond Bar with the carrier shown. If it isn't, check the California page or look the number up.
| Prefix | Carrier | Type | Assigned |
|---|---|---|---|
| 840-240 | Onvoy | Internet / business phone carrier | 2022 |
| 840-244 | T-Mobile (including Sprint) | Mobile carrier | 2023 |
| 840-289 | Onvoy | Internet / business phone carrier | 2026 |
| 840-588 | Fractel | Internet / business phone carrier | 2021 |
| 909-274 | Charter / Spectrum | Internet / business phone carrier | 2009 |
| 909-323 | Onvoy | Internet / business phone carrier | 2016 |
| 909-345 | Bandwidth | Internet / business phone carrier | 2012 |
| 909-348 | Level 3 (Lumen) | Internet / business phone carrier | – |
| 909-378 | MCImetro (Verizon Business) | Internet / business phone carrier | 2013 |
| 909-396 | Frontier | Local phone company | – |
| 909-444 | Frontier | Local phone company | – |
| 909-468 | Frontier | Local phone company | – |
| 909-551 | Charter / Spectrum | Internet / business phone carrier | 2014 |
| 909-569 | Teleport Communications America | Internet / business phone carrier | – |
| 909-594 | Frontier | Local phone company | – |
| 909-595 | Frontier | Local phone company | – |
| 909-598 | Frontier | Local phone company | – |
| 909-610 | MCImetro (Verizon Business) | Internet / business phone carrier | – |
| 909-612 | Frontier | Local phone company | – |
| 909-616 | AT&T Mobility | Mobile carrier | 2018 |
| 909-655 | Bandwidth | Internet / business phone carrier | 2015 |
| 909-718 | Tpx Communications | Internet / business phone carrier | – |
| 909-760 | Onvoy | Internet / business phone carrier | 2016 |
| 909-839 | Frontier | Local phone company | – |
| 909-859 | AT&T (local phone) | Local phone company | – |
| 909-860 | Frontier | Local phone company | – |
| 909-861 | Frontier | Local phone company | – |
| 909-869 | Frontier | Local phone company | – |
| 909-895 | Level 3 (Lumen) | Internet / business phone carrier | – |
| 909-914 | Bandwidth | Internet / business phone carrier | 2020 |
| 909-978 | Tpx Communications | Internet / business phone carrier | – |
| 909-979 | Frontier | Local phone company | – |
Most prefixes are pooled: the 10,000 numbers in each are handed out in blocks of 1,000 to different carriers, and people can also take their numbers to another carrier. So the carrier shown is where the prefix started, not a guarantee about any one number. Source: NANPA central office code records, 2026-10-10.