Phone numbers become unavailable in certain NPA (Number Plan Area), also know as area codes, due to the phenomenon known as "number exhaustion." This situation arises when almost all possible phone number combinations within an area code have been assigned. Each area code can only accommodate a finite number of phone numbers, and with increasing population and the proliferation of mobile devices and businesses requiring unique numbers, the demand often outstrips the supply. This leads to the exhaustion of available numbers in popular or densely populated area codes, making it challenging to obtain new numbers within these areas.

To address the issue of number exhaustion, telecommunications authorities implement a solution known as an "overlay." An overlay involves introducing a new area code to the geographical region already served by an existing exhausted area code. This means that the same geographic area is covered by two or more area codes, significantly increasing the pool of available phone numbers. Unlike splitting a region into multiple area codes, overlays allow existing phone numbers to retain their original area codes, while new numbers are assigned the new overlay codes. This approach effectively expands the numbering resources without disrupting existing phone number assignments.

Refer to this list to find exhausted area codes.