Area codes 602, 480, and 623

Area codes 602, 480, and 623 are telephone area codes in the North American Numbering Plan (NANP) for most of the Phoenix metropolitan area in the U.S. state of Arizona.

Area code 602 is the oldest area code in Arizona and was assigned in 1947 for the entire state. Under pressure from population growth and new telecommunications services, the numbering plan area (NPA) was reduced twice in five years in the 1990s. In 1995, the state outside metropolitan Phoenix was split off with area code 520. In 1999, a second split created two new area codes: 480 in the East Valley and 623 in the West Valley. Metro Phoenix continued to be a single rate center after the split, so that calls between the three area codes were generally local calls.

By the early 2020s, 480 and 602 were facing exhaustion within the decade, but 623 continued to have hundreds of unassigned central office codes and was not expected to exhaust for the foreseeable future. As a result, the Arizona Corporation Commission, which regulates public utilities, approved a plan in 2021 to reverse the 1999 split and convert 602, 480, and 623 into overlay codes for the entire Phoenix area in 2023. The creation of the three-code overlay complex made ten-digit dialing mandatory across the Valley; it was already required in 480.

Early history and split of 520
When the American Telephone and Telegraph Company (AT&T) created the first nationwide telephone numbering plan in 1947, Arizona was designated as a single numbering plan area and received a single area code, 602, of the original 86 area codes for routing telephone toll calls between states.

Despite Arizona's explosive population growth in the second half of the 20th century, 602 remained Arizona's sole area code for 48 years. However, as early as 1988, Mountain Bell, the incumbent local exchange carrier in the state, believed Arizona was growing too quickly to remain a single numbering plan area and requested a second area code for Arizona. BellCore, which at the time administered the assignment of area codes, denied the request and instead placed Arizona into the first phase of interchangeable dialing, in which central office codes with a middle digit of 0 or 1 were made available for use. This meant that in-state toll and collect calls would require dialing the area code.

By the early 1990s, the need for a new area code could no longer be staved off. In 1993, Arizona was allocated a second area code, area code 520, for all of the state outside the Phoenix metropolitan area. 520 was introduced on March 19, 1995. Permissive dialing of 602 continued across Arizona until October 22, 1995. On that date, use of 520 became mandatory for rural Arizona. The new area code became mandatory in Flagstaff, Prescott, and Yuma on June 30, 1996, and in Tucson on December 31, 1996. The freed central office codes in 602 were then used for new telephone numbers in the Phoenix area.

Overlay or split?
The creation of 520 was originally intended as a long-term solution; under original projections, Arizona was not expected to need another area code until at least 2015. However, Arizona's explosive population growth in the 1990s, the introduction of new competitive telephone service providers and telecommunications technologies (such as cell phones, pagers, and dial-up Internet), and an inefficient number allocation system brought 602 to the brink of exhaustion far sooner than expected. It soon became apparent that metropolitan Phoenix, now one of the largest toll-free calling zones in the nation, needed at least one additional area code. By 1997, two ideas were on the table for relieving exchanges in metropolitan Phoenix, and the telephone industry could not reach a consensus on which was more suitable. US West, formerly Mountain Bell, was in favor of an overlay, in which a second area code would be added to the existing 602 area. This would have required the implementation of ten-digit dialing for all local calls. The other option was a split, in which the suburban portion of the Valley would have received another area code, with 602 retained by most of the city of Phoenix. Conversely, newer entrants to the telephone market, like MCI Communications, supported a split because US West, as the dominant provider in the region, held most of the numbers in 602.

The final word rested with the Arizona Corporation Commission (ACC), which regulates public utilities. In November 1998, on a 2–1 vote, the ACC voted to adopt the overlay for implementation in 1999. Corporation Commission staff felt the overlay offered a more long-term solution than a split, which was projected to require additional relief within four years for metro Phoenix and 12 years for suburban areas. In early December, 480 was assigned as the second area code.

The adoption of the overlay plan, however, met with criticism from the public. Overlays were still a new concept at the time and met with some resistance due to the need for ten-digit dialing. Just two weeks after voting in favor, the ACC opted to reconsider in a move that clearly favored the adoption of a split. The "doughnut" split gained two wrinkles in the process. First, the commission opted to consider putting north Phoenix in the new area code as well. Second, the idea of changing to a three-way split where the East Valley and West Valley areas being spun out from 602 would receive their own area codes gained traction.

On December 18, 1998, the Corporation Commission approved the final plan to go into effect on September 1, 1999: a three-way split. The city of Phoenix minus Ahwatukee and areas north of Union Hills Drive remained in 602, along with slivers of Tempe and Glendale. Most of the East Valley, along with Town of Paradise Valley and north Phoenix east of 22nd Street, took area code 480. Most of the West Valley, plus all of Phoenix north of Union Hills, was placed into area code 623. (556 was also considered instead of 623. ) The three-way split took place even though, in actuality, just three million phone numbers had been issued in 602, less than half of the 7.5 million numbers available. Permissive dialing of the new area codes started on April 1.

The three Valley area codes formed one of the largest local calling areas in the western United States. With the exception of the slivers of the Valley that are in the 520 and 928 area codes, no long-distance charges are applied from one portion of the Valley to another. Even with the split into three area codes, much of the Valley was still part of the Phoenix rate center.

Boundary elimination overlay
The three-way split, combined with the implementation of number pooling and other practices to encourage efficient use of telephone numbers, gave the Valley enough telephone numbers to absorb more than 20 years of growth. By October 2020, the North American Numbering Plan Administrator (NANPA) forecast that area code 480 would be exhausted by the first quarter of 2024 and 602 two years later in 2026. However, 623 was not forecast to exhaust until 2069; in 2021, it had 299 assigned central office codes as opposed to more than 700 in each of 480 and 602.

After NANPA initiated relief planning for 480, the state's telecommunications industry recommended to the Corporation Commission that the 1999 area code boundaries be eliminated, creating a three-code overlay complex. This would allow for pooled numbers to be used anywhere in the metropolitan area and for the assignment of new numbers (primarily from 623, as it had most of the unassigned codes) throughout the single rate center. By this time, overlays had become the standard for relief. No area codes had been split anywhere in the United States since 2007, and it was not possible to split the 480 area code because of the 2021 implementation of 10-digit dialing there. This would save the assignment of two area codes compared to individual all-service distributed overlays of 480 and 602 and last 26 years, as opposed to 35 for the introduction of new area codes. The Corporation Commission approved this plan on November 9, 2021. Implementation began in August 2022, after the national deadline to activate 988. A six-month permissive ten-digit dialing period for the 602 and 623 areas commenced on February 11, 2023, ahead of the in-service date of September 12, 2023, of the overlay. As ten-digit dialing had already been in use in 480, 520, and 928, the boundary elimination made it mandatory statewide.