Located in the Central (GMT -06:00) time zone. Area code 331 serves the state of Illinois. This area code was assigned on July 23rd, 1999. On October 7th, 2007 it went into service. 331 is a General Purpose Code. 331 service area does overlap with 630 area codes. Dialing pattern for local calls within the same NPA (area code) is 1+10D.