Typically when you sign up for a mobile line in the US the carrier will assign you a number whose area code (first three digits) is "correct" for your address.

Typically when you sign up for a mobile line in the US the carrier will assign you a number whose area code (first three digits) is "correct" for your address. But it's not strict, and you can keep that number anywhere you move after that. I got my first cell phone when I was living in central Missouri, so my area code is 573 even though I no longer have any ties to that area. Identifying this type of robocall is very easy for me.