Before more advanced computerized call routing and SS7 signaling changes area codes had to map to one specific non-overlapping region. So when a metro area need

Before more advanced computerized call routing and SS7 signaling changes area codes had to map to one specific non-overlapping region. So when a metro area needed more exchange prefixes they had to split the area code and some subset of customers got pushed onto a new area code. In more modern times they assign an "overlay": multiple area codes can be assigned to the same region so there is no need for splits. Actually the system isn't really hierarchical anymore. Thanks to number portability optimizations any switch can lookup the destination switch for a 10 digit number. Thanks to faster computers and larger storage that isn't the problem it once was.