The Subscriber Extension

The last sequence in the numbering plan is the subscriber extension number. This is a four-digit sequence that can use any digit in all of the slots, allotting 10,000 customer telephone numbers in each of the exchange codes. Because the four digit sequence can be composed of any numbers, the intent is to give every subscriber their own unique telephone number. This hasn’t changed as yet. However, the possibility that we can still run out of numbers always exists. Therefore, a couple of ideas have been bandied around: add two, three, or four more digits to the end of the telephone numbering plan or add one or two more digits to the area code or exchange code.

In either method, users will be asked to dial more digits, an idea that is never popular, but might become a necessity. However, this is far more complicated than just adding a few more digits here and there. The whole world will be impacted by any such decision and the length of time to implement such a global change will be extraordinary.