Despite western Washington's growth in the second half of the 20th century, this configuration remained in place for 38 years. By the start of the 1990s, however, 206 was nearing exhaustion of central office codes from proliferation of cell phones, pagers, and fax machines.
On January 15, 1995, most of the old 206 territory outside of the Seattle/Tacoma area was split off with area code 360, which was one of the first two area codes not conforming to the then-traditional N0X/N1X format.[1]
Numerous residents in the Seattle exurbs protested about no longer being associated with 206, leading US West, now part of Lumen Technologies, to return these to 206 shortly after the split. As part of the reintegration, the cities of Des Moines and Woodway were both split between the new area codes, required by the capacity of the switching centers.[2]
However, western Washington would have likely needed another area code before the end of the decade, as 206 was still on the brink of exhaustion even after the 360 split. The return of the Seattle exurbs, combined with the continued proliferation of cell phones, faxes, and pagers, hastened a three-way split of the 206 territory, effective on April 25, 1997. The southern portion, including Tacoma, received area code 253, while the northern portion, including Everett and the Eastside, was assigned area code 425.[3][4]
On June 15, 2023, the WUTC approved the expansion of the overlay plan of area code 564 to the 206 area before central office code exhaustion occurs in 2025.[5] Area code 564 had only served the 360 numbering plan area since 2017. 564 will also be added to the 253 and 425 area codes once necessary.[6][7] However, ten-digit dialing has been mandatory for all of Washington west of the Cascades since 2017.
