Think of it this way.
You live near the state line, which is 1/2 mile away.
In Kansas City, KS you have a different area code than someone who lives in Kansas City, MO and is only 1 mile east of you.
If both of you enter only the 7 digit phone number, they can only call you IF they happen to be on your side of the state line.
likewise, you can only call them, if you move across the stateline and then call them....
the cellsites look at your GPS location, and if you are out of the area, they send you over to a different cell site, or drop the call completely.
This is why, it is so critical that you include the Area code on all phone numbers. To not do so, is "dereliction of duty" in military terms.
AT&T is famous for simply dropping all calls when you cross over state lines.
I am all too familiar with that, my friend is a truck driver, and every time, he crosses a state line, his AT&T phone drops our call.