Please see future.html for more information on the projected exhaust of the current NANP 3-3-4 number format.

One common proposal for expanding beyond 10-digit numbers would simply expand the supply of area codes by moving to 4-digit area codes, keeping the current 7-digit local number format. All area codes with ‘9’ as the middle digit have been reserved for expansion, so the obvious proposal is to migrate to 4-digit codes by adding a ‘9’ as the second digit of existing area codes.

I propose, however, to take the expansion a step further, since a tenfold increase might last only a few decades before yet more expansion is required. I propose that, in tandem with inserting the 9 into each NPA, we prepend a 3 onto each CSOD, creating a 12-digit number. Permissive dialing would still be possible — if the second digit of the dialed number is a 9, then it is a 12-digit number; otherwise, it is an old 10-digit number. I selected 3 as the prefix for the CSOD to allow the simple slogan, “Remember to dial 12 digits: If you only have 10 digits, add 9 and 3 to get 12!”

This proposal would expand the numbering capacity of the North American Numbering Plan by a factor of 100, which should allow ample room to assign a telephone number to every person, computer, fax machine, pet, and household appliance. A twelve-digit numbering space would allow more than 500 billion possible numbers.

Some proposals for 8-digit numbers simply add a digit at the end of existing 7-digit numbers, so that 555-0123 becomes 555-01230. Adding the second extra digit to the CSOD rather than at the end of the number has the advantage that it does not magnify the inefficiency in number utilisation caused by the requirement that each town have its own prefix, since there will now be nearly 8000 (or possibly 10,000) CSODs in each NPA, but each CSOD will still represent only 10,000 SLIDs. A further advantage is that the introduction of 12-digit numbers will usher in a lengthy period of stability in NPA assignments, since it will take years for even the fastest-growing NPA to absorb a tenfold increase in capacity.

It is possible that by the time the next expansion becomes necessary, full national number portability may have been implemented, meaning that an end-user could retain a given number moving across town, changing telephone carriers, or even moving across the country. Such a system of number portability would provide even greater flexibility, since new numbers could be assigned from any area code with spares available, and there would be far fewer numbers left on referral to new numbers or error recordings. Thus, a ten-fold increase in capacity might be adequate to last for decades or even centuries; then again, why not go for the 100-fold increase and be certain? In particular, there are serious technical, billing, and human-factors issues remaining to be resolved before nationwide number portability becomes a reality.

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