Instead of reassigning local numbers a new area code, the introduction of a new area code to the same region introduces a new pool of more than 9 million numbers. A new area code is first contemplated for a region even when all numbers in the existing area code region are not yet depleted. The measurement applied is based on the size of the available pool of numbers that can still be assigned. As that pool dries up, it nears exhaustion. And each local telephone company responsible for administering the numbering within their region defines exhaustion differently.
Exhaustion can come one or two years before the pool of available numbers is physically depleted.
In most cases, exhaustion of an area code is the realization that the resource no longer has the capacity to fully address service needs. In the case of our corporate customers, thousands of numbers can still be out there because they are not being used. But they are unavailable because they have been taken out of circulation as part of the legitimate two or three year growth plan of the company. Sufficient numbers in large enough blocks are no longer available. In any case, the declaration of coming exhaustion is the telltale sign that change and a new area code is in the air.