Area Code 331 – Illinois
The phone area code for Illinois is 331 (international format +1331). Browse local exchange codes (NXX) and carrier assignments below.
📞 Exchange Codes (NXX) for Area Code 331
Select a central office code to view local carrier data and detailed information
📍 Geographic Location of Illinois
Geographic Coverage and Jurisdiction
Area code 331 is a geographic North American Numbering Plan Area (NPA) code serving the state of Illinois, specifically the western and central suburbs of the Chicago metropolitan area. It is an overlay code for the existing area code 630, which means both codes serve the same geographic region. The Illinois Commerce Commission (ICC) approved the overlay to ensure an adequate supply of telephone numbers for the growing population and business demand. The region covered by 331 includes communities such as Naperville, Aurora, Elgin, Wheaton, Downers Grove, and Joliet, among others. These areas are part of the Chicago-Naperville-Elgin metropolitan division, one of the most populous in the United States.
Dialing Formats and Procedures
Because 331 is an overlay, all local calls within the 630/331 region require 10-digit dialing. This means callers must dial the area code plus the seven-digit local number for all calls, even if the call is to a neighbor across the street. For example, to call a business in Naperville from within the same region, you would dial +1 331 555 0123 or 1 331 555 0123 (the leading '1' is used for domestic long-distance calls within the U.S.). For international callers, the format is +1 331 XXX XXXX.
Caller Instructions
For local callers within the 630/331 area: simply dial the 10-digit number (area code + local number). For callers from other parts of the United States: dial 1 (trunk prefix) followed by the 10-digit number. For international callers: dial the international access code (e.g., 011 from many countries) or the '+' sign, then 1 (U.S. country code), then 331, then the seven-digit local number. Remember that the area code is essential for routing; omitting it will result in a failed call.
Overlay Considerations
The 331 overlay was introduced in 2007, and since then, all new telephone lines in the 630 region may be assigned either 630 or 331. This means that two numbers in the same town can have different area codes, but they are still local calls. There is no need to change existing numbers, and no per-call charges apply for calls between 630 and 331 numbers. The overlay ensures that the region has sufficient numbering resources for future growth, including advanced services like VoIP and mobile communications.
Practical Examples
- To call a friend in Aurora from outside the U.S., dial +1 331 456 7890.
- To call a local business in Naperville from a landline in the same area, dial 331 555 0123.
- To call from a mobile phone in Chicago (area code 312) to a 331 number, dial 1 331 555 0123.