Area code 331: Illinois
Area code 331 is a US state area code for Illinois, on Central Time. It has 222 prefixes in service across 24 rate centers, the largest being Aurora, Naperville and Downers Grove. It shares its area with 630, so local numbers there can start with any of them. Local calls there take 1 + 10 digits.
Local time in area code 331
America/Chicago
Prefixes in service
222 of 792 possible
24 rate centers
Area code 331 facts
| Location | Illinois, United States |
|---|---|
| Time zone | Central Time |
| Overlays | 630 |
| Overlay of | 630 |
| In service since | October 7, 2007 |
| Local calls | 1 + 10 digits |
| Carrier | Not shown. An area code is where a number was issued; numbers port between carriers. Check line type |
Which cities and towns use area code 331?
NANPA records each prefix against a rate center, the local billing area a number belongs to, usually named after its main town. These are the 331 rate centers with the most prefixes in service. 12 smaller ones aren't listed.
| Rate center | Prefixes |
|---|---|
| Aurora | 45 |
| Naperville | 33 |
| Downers Grove | 21 |
| Saint Charles | 17 |
| Lombard | 13 |
| Elmhurst | 10 |
| Lemont | 10 |
| Bartlett | 8 |
| West Chicago | 8 |
| Oswego | 7 |
| Wheaton | 7 |
| Geneva | 7 |
Source: NANPA central office code assignments, file dated 10/17/2026. Each prefix holds up to 10,000 numbers.
How do I dial a 331 number?
NANPA's dialling plan for 331: local calls inside the code take 1 + 10 digits, toll calls 1 + 10 digits. From another area code, dial 1 first. From abroad, use +1. The examples use 555-0147, from a range reserved for fictional numbers.
| Local call inside 331 | 1-331-555-0147 |
|---|---|
| From another US or Canadian area code | 1-331-555-0147 |
| From outside North America | +1 331 555 0147 |
| E.164, for CRMs and SMS tools | +13315550147 |
When did area code 331 start?
Area code 331 went into service on October 7, 2007 as an overlay of 630. Today 331 and 630 cover the same area as an overlay, which is why 10-digit dialling is needed for local calls there.
Tools for 331 numbers
- Area code lookup
Look up any code or a whole list of numbers, or find the codes for a state or city.
- Area code time zone
Local time for a list of numbers, with a callable-now flag.
- NPA-NXX lookup
Rate center and state for one prefix, a level below the area code.
- TCPA calling hours checker
Whether it's inside the federal 8 a.m. to 9 p.m. calling window there now.
Next step
A 331 number tells you where it was issued, not what it is now.
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Area code 331 questions
- What state is area code 331?
- Area code 331 is in Illinois, United States. Its largest rate centers by number of prefixes are Aurora, Naperville, Downers Grove, Saint Charles and Lombard.
- What time zone is area code 331 in?
- Central Time.
- What area codes overlay 331?
- 630. An overlay adds a new code to the same area instead of splitting it, so neighbours can have different area codes and local calls need 10 digits.
- When did area code 331 start?
- Area code 331 went into service on October 7, 2007 as an overlay of 630.
- Is a 331 number a cell phone or a landline?
- The area code can't tell you. In the US and Canada mobiles, landlines and VoIP lines share area codes and prefixes, and numbers move between carriers and line types. Only a live line-type lookup answers it for one number.
- How many phone numbers does area code 331 have?
- 222 of its 792 possible prefixes are in service, and each prefix holds up to 10,000 numbers. That counts prefixes assigned to carriers, not numbers in use.
Other area codes in Illinois
Sources
Location, overlays, in-service date and dialling plan: NANPA NPA database, file dated 10/16/2026. Prefixes and rate centers: NANPA central office code assignments, 10/17/2026. Time zones mapped to the IANA time zone database. By the SimpleVerifier Team; how we build these pages.