A phone number in USA without international dialing sequence. A phone number in USA is a sequence of $10$ digits $(0-9).$ The digits are divided into $3$ groups: a $3-$digit area code, a $3-$digit prefix $($also called the exchange code or central office code$),$ and a $4-$digit suffix $($also called the carrier number, line number, or station code$).$ The area code and the prefix will never start with $0$ or $1,$ but the suffix can. To display a phone number, we can put a pair of parentheses$3$ around the area code; leave a space between the closing parenthesis and the prefix; and put a hyphen between the prefix and suffix. $($There are many other commonly used formats, but we focus on this format only.$)$ For example, the main phone number for FSU is $(910)672-1111$