825 in Alberta (over 403 and 780). In an phased example, area code 564 was implemented initially only on area code 360 in 2017, but will expand to three other codes when needed, in western Washington. A boundary-extension overlay is a plan in which one or more neighboring area codes (an overlay code or a single primary area code) is expanded to serve the area as well. Examples include 321 over 407 in central Florida and 778 over 250 in British Columbia. In 1999, 281 was the first to do so, merging with 713 in Houston; later that year, 972 merged with 214, followed by 323 merging with 213 in Los Angeles in 2017 and 858, in San Diego, with 619 a year later. In 2023, the first multiple boundary-extension overlay will take place in the Phoenix metropolitan area with the folding of area codes 602, 480 and 623 into a single overlay area. Service-specific overlays are not practiced in the NANP anymore. Only one case, the first overlay area code in the NANPA, 917, was an example of this. It was established originally as an area code specifically for cellphones and pagers in New York City, but soon afterward, the FCC specified that area codes must not be service-specific, but it grandfathered this use for 917. Later, 917 became available for landline assignments in New York City on a limited basis. Number pool management The persistent unpopularity of new area code creation by split or overlay plans has led to a change in the rules of number block allocation to conserve the pool of available telephone numbers. The change, which allowed for the assignment of smaller number blocks, is commonly known as number pooling and has noticeably slowed the need for area code growth. For example, the western half of Washington, including Seattle, narrowly avoided the need for an overlay in 2001. Area code 564, originally planned for introduction in October 2001, was delayed indefinitely in August 2001 after state regulators determined that the use of number pooling had allowed existing numbers in western Washington to be used more efficiently. The 564 overlay plan did not have to go into action until 2017. Number pooling is not practiced in Canada. As was previously mentioned, every competing carrier is issued blocks of 10,000 numbers for every rate center in which it offers service, regardless of its actual subscriber count. Individual