Occasionally different parts of the country grow so much and have so many local phone numbers that changes are needed. Sometimes new area codes are introduced and in some places people have to dial their local area code to complete a call. These changes are overseen by the North American Numbering Plan Administration (NANPA) which is overseen by the Federal Communications Commission (FCC).
Several new area code overlays will take effect in the second half of 2019. When details are available, you will find them here.