Why do area codes shift?

Answer 1

An area code has a limited quantity of telephone numbers, and more particularly a limited quantity of blocks of numbers. When an area code runs low on available number blocks, there are only a few options. In many countries, telephone numbers can get longer by adding a digit, but that is not possible in the US or Canada. The existing area code can split, with about half of the numbers changing to a new area code, or it can have an overlay, in which a new area code serves the same geographic territory.

There are also rare instances in which the boundary between two existing area codes is changed. For example, the town of Dixon, California, changed from 916 to 707 rather than be split into 530.