North American Numbering Plan expansion
The expansion of the North American Numbering Plan (NANP) is an anticipated requirement to provide more telephone numbers to accommodate future needs beyond the pool of ten-digit telephone numbers in use since the inception of the NANP in 1947. A 2017 analysis estimated that the current numbering system would not be exhausted until after the year 2047. 
The North American Numbering Plan was devised in the 1940s, implemented in 1947, and has provided customer direct-dialed long-distance telephone service since 1951. Occasionally, the growing demand for telephone numbers has mandated certain changes in the format of telephone numbers and method by which they are allocated to telephone companies. Instead of expanding the size of telephone numbers, as implemented in many countries outside North America, the NANP was modified to allow continued use of seven-digit telephone numbers and a three-digit area code. The first modifications removed restrictions placed on the central office prefixes that existed due to the use of central office names (exchange names) in the numbering plan. At first, telephone companies would resort to area code relief if the 600 available combinations were almost used up, even when 40 more were available if assigned as all-numeric or using letter combination that were considered unpronounceable (55, 57, 95 and 97). The change to all-number calling ended this restriction.
Later modifications relaxed the restrictions for the middle digit of the central office prefix, allowing them to resemble the area code format. This modification was noticeable in the largest U.S. cities during the early 1980s, and became mandatory after 1988, shortly after the 508 area code was assigned in Massachusetts. Finally, in 1995, area codes were freed from the restriction of having only 0 or 1 as the middle digit.
Every place in the countries and territories of the NANP already belongs to a numbering plan area (NPA). Assigning a new area code requires either geographically splitting an existing numbering plan area (split plan) and reassigning the subscribers in the new, smaller area to a new area code, or assigning an additional area code to an existing numbering plan area, which is called an overlay plan. The first new area codes were geographic splits, until the geographical areas were at risk of becoming unreasonably small. Since then, most new area codes are overlaid. When a numbering plan area has more than one area code, telephone companies usually can no longer support dialing seven-digit local numbers, and require ten-digit dialing for all calls within the geographical area. Universal ten-digit local numbering is arguably inevitable, as more geographical areas get overlaid.
Ten decimal digits allow a maximum of 10 billion telephone numbers. The numbering rules of the plan reduce that to 6.4 billion telephone numbers. However, a considerable percentage of these numbers will remain unused when the last available NPA code is assigned, because thousands of numbers will be reserved in exchanges that serve only small population centers, 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.
Expansion options have been discussed 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 2017 analysis anticipates exhaustion after 2047.
In the usual NANPA 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 first three digits of ten are referred to, by industry jargon, as the NPA, the next three as the NXX or CO code (central office code), and the last four as the line number.
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 NXX, 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. This distinction would quickly vanish as digits other than 0 and 1 are used in these positions after permissive dialing ends.
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, it is assumed 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 to 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 because 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.
A disadvantage to expanding to 11 digits under this plan is that area codes would continue to be "consumed" at the present rate.
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. A potential 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 middle block could theoretically be any digit.
An advantage of expanding to 12 digits under this plan is that the area codes would be "consumed" at a much slower rate, as there would be ten times as many possible combinations in each area code.