Connecticut currently utilizes 4 area codes. When first established by AT&T and the Bell System in 1947, 203 covered the entire state of Connecticut. The most recently added was 959 in 2014 as an overlay to 860. Today, 860 is the most populated with an estimated 1.8 million of the state's 3.6 million residents.
Sources: North American Numbering Plan Administration (NANPA), telecom providers, and the U.S. Census Bureau.