I remember being confused when I first arrived in the U.S. because the phone number system is different from Korea.

I wondered why there are two area codes and why I have to dial all 10 digits for local calls. Living in New Haven, I've gotten used to seeing numbers that start with 203 every day, but initially, I definitely had questions. Today, I'll organize the local phone number system based on data.

The main area code for New Haven is 203. 203 was one of the first area codes designated when the North American Numbering Plan (NANP) was established in 1947. Initially, 203 covered the entire state of Connecticut. In 1995, as Connecticut's population and communication demands grew, the northern and central parts of the state were separated into the 860 area code. As a result, 203 became responsible for southwestern Connecticut, which includes New Haven and Fairfield County. Major cities in southern Connecticut, such as New Haven, Bridgeport, Stamford, Waterbury, Norwalk, and Danbury, all fall under the 203 area code.

However, in 2009, the 475 area code was added. This is an overlay code for 203. It means that two area codes apply simultaneously to the same geographic region. It was designated as a backup in case the 203 numbers ran out. Since 203 and 475 cover the same area, some people in New Haven have numbers starting with 203, while others have numbers starting with 475. There's no better or worse number; it's just a different number assigned.

There's an important rule regarding calling. Even for local calls within the 203/475 area, you must dial all 10 digits. This means you have to dial either 203-XXX-XXXX or 475-XXX-XXXX in full. You can't just dial 7 digits like before. However, unlike in Korea, you don't need to dial 1 before local calls. You can just start with "203" instead of "1-203." When making long-distance calls or connecting to other area codes, you do need to dial 1 first. The time zone is Eastern Standard Time (EST, UTC-5), and during the summer, it changes to Eastern Daylight Time (EDT, UTC-4). The clocks move forward one hour on the second Sunday in March and back one hour on the first Sunday in November. When calling Korea from New Haven, you need to consider this time difference, which is about 13 hours during daylight saving time and 14 hours in winter.