[Motel75]

Another reason is that US numbers are, or used to be scarce, due to the fixed number of digits and geographical location of the numbers. This means that, unlike elsewhere, a carrier cannot simply add another digit or come up with another prefix and create another hundred million theoretical subscriber numbers. With area codes running out of numbers in the US, a phone number has a theoretical value itself. However, recent changes, particularly assigning unused numbers to carriers in smaller blocks, have freed up capacity and reduced the pressure for more area codes.