North American Numbering Plan expansion
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The North American Numbering Plan has been in use since 1951, and has endured that time with only minor changes. So far, unlike many countries outside of North America, it has not been necessary to lengthen phone numbers to accommodate expansion. To date, the accommodation of demand has been achieved by adding area codes (NPAs) for relief (either as a split or as an overlay plan), a few isolated instances of rate centre consolidation and (in the US) number pooling which allowed the reclamation of many unused numbers. The addition of area codes has required ten digit dialing for some local calls, and dialing 1 before a long-distance call in order to distinguish identical number sequences in local exchange prefixes and area codes.
Although the plan provides for a potential of 6.4 billion telephone numbers, a considerable percentage of these numbers will remain unused when the last available NPA code is assigned, as thousands of numbers will not be in use in exchanges that serve only a small population center, the exchange being served by a single NPA-NXX combination in non-competitive markets. More combinations would be partially unused in the event that a small market has competitive providers.
As a result, as the supply of unassigned NPA codes dwindles, it will be necessary for telephone carriers in North America to implement expansion beyond the 10-digit format. Expansion has been a matter of discussion by industry forums for several years, and although a recommendation has been made, that expansion plan format has not been approved by the regulatory authorities, i.e. the Federal Communications Commission in the United States and the Canadian Radio-Television and Telecommunications Commission and the regulators may choose a different expansion plan. The industry forum considered dozens of potential options and identified the difficulties with each option.
NANPA regularly performs exhaustion analyses. The 2014 analysis anticipates exhaustion after 2044.
Current industry recommendation
In the usual phone number jargon, the letter N represents a numeral from 2 through 9, while the letter X represents any numeral. Thus an NXX number may be any from 200 through 999, while XXXX would be 0000 through 9999.
The telephone industry's current recommendation assumes first that mandatory dialing of all ten digits is required to complete telephone numbers, even for a local call, throughout the North American Numbering Plan area, which includes many Caribbean and Pacific territories and nations.
The plan proposes the insertion of 00 or 11 between the NPA and the Central Office Code (NXX in industry jargon), to produce 12-digit numbers. The plan further proposes that the US would use either 00 or 11, while Canada would use the other, in order to allow customers to distinguish countries by use of these digits, which do not appear at the beginning of the 12-digit number.
Under this proposal, the N9X format NPA codes, which are currently reserved from assignment, would be released and be available for normal assignment for code relief and other purposes.
For these examples, we will assume that the new digits will be '00' for the US, and '11' for Canada. With these assumptions, under this plan, the New Jersey telephone number 609-555-0175 would become 6090-0555-0175, and would be dialed as such. Likewise, the Ontario number 613-555-0175 would become 6131-1555-0175.
One advantage is that during the transition period, permissive dialing could be enabled. This means that until everyone has adjusted to the new dialing system, users would still be able dial the shorter, 10 digit numbers. Since currently, the 4th digit (or digit 'D') cannot be 1 or 0, if the telephone system detects 1 or 0 in the 4th position it will process the number as a new 12 digit number, and if it is any digit other than 1 or 0, it can process it as an existing 10 digit number until the transition is complete.
Proposals that utilize the reserved N9X format codes for expansion include the following proposals:
N9XX, with no change to the remainder of the phone number
This proposal would expand numbers to eleven digits overall. A 9 would be inserted as the new second digit of all area codes (e.g. 212 would become 2912, 916 would become 9916). Permissive dialing would be allowed, as exchange equipment, on detecting a 9 as the second digit of the area code, would respond appropriately to expect 11 digits, or 10 in the absence of a 9 in that position.
Under this plan, the New Jersey telephone number 609-555-0175 would become 6909-555-0175.
N9XX, with a new initial digit
With a new initial digit in front of the last seven digits of the phone number, this proposal would expand numbers to twelve digits overall. As with the above plan, a 9 would be inserted as the new second digit of all area codes. The problem would occur with permissive dialing of local calls where the area code is not presently required (areas with no overlay in effect). If the added digit were 3, for example, numbers that already begin with a 3 would present a problem, probably resolved using either a "time-out" if the customer only dials seven digits, or a flash-cut to mandatory eight digit dialing.
Under this plan, the New Jersey telephone number 609-555-0175 could become 6909-3555-0175, although the added '3' in the '3555' could theoretically be any digit.
-  April 2014 NANP Exhaust Analysis