Area code 331: where it is and who calls from it
Area code 331 is in Illinois.
Illinois (Aurora, Naperville, Oswego; western suburbs of Chicago)
| Where | Illinois |
|---|---|
| Time zone | Central Time ยท local time |
| In service since | 2007 |
| Overlay | Yes, with 630 |
| Prefixes assigned | 224 of 792 possible (28%) |
Overlay: 331 covers the same area as 630. Overlay areas use 10-digit dialing, so a seven-digit number alone won't connect, and neighbors can have different area codes.
Where 331 numbers started
Every prefix is assigned to a rate center, the local calling area it was set up in. These are the biggest in 331, with how many prefixes each has.
Who holds 331 numbers
- Mobile carrier 46%
- Local phone company 1%
- Internet / business phone carrier 51%
- Phone company 2%
| Carrier | Type | Prefixes | Share |
|---|---|---|---|
| Onvoy | Internet / business phone carrier | 44 | 20% |
| T-Mobile (including Sprint) | Mobile carrier | 42 | 19% |
| Verizon Wireless | Mobile carrier | 33 | 15% |
| AT&T Mobility | Mobile carrier | 26 | 12% |
| Fractel | Internet / business phone carrier | 18 | 8% |
| Bandwidth | Internet / business phone carrier | 11 | 5% |
| TON80 Communications | Internet / business phone carrier | 9 | 4% |
| Level 3 (Lumen) | Internet / business phone carrier | 6 | 3% |
| Peerless Network Of Illinois | Internet / business phone carrier | 5 | 2% |
| MCImetro (Verizon Business) | Internet / business phone carrier | 4 | 2% |
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.
States served: Illinois.
Other area codes in Illinois
217 224 309 312 447 464 618 630 708 730 773 779 815 847 861 872
Got a call from 331?
An area code tells you where a number was first assigned, not where the caller is. People keep their numbers when they move, and scammers can fake any number. If you weren't expecting the call, let it go to voicemail.
If the call shows a 331 number that matches the first six digits of your own, that's often neighbor spoofing. Use the box above to check the full number.
Area code facts from the NANPA area code database (updated 10/06/2026); prefixes and carriers from NANPA's central office code records (updated 2026-10-10). Place description adapted from Wikipedia, CC BY-SA 4.0.