to assign multiple area codes to a single numbering plan area. An area code overlay required additional technology in all affected switching systems to recognize the local area codes and select the appropriate routing for local calls, even within the wire center. This eliminated the possibility of maintaining seven-digit dialing, but required the dialing of ten digits for all calls. Consumer groups and some state regulators, such as the Illinois Commerce Commission and Citizens Utility Board for northwest Chicago, and the NYS Public Service Commission in NYC, opposed the requirement with attempts at litigation. The added effort in remembering and dialing longer numbers damaged the popularity of overlay plans, which were introduced as a means to reduce the inconveniences associated with the traditional method of splitting an area. However, overlay plans have become the primary relief method and ten-digit dialing has become increasingly common in the U.S. and Canada. 1+10 dialing Long-distance calls within a numbering plan area may also require dialing of an extra digit. Eleven digits for toll calls became standard in all of North America by the end of 1994 to allow introduction of "interchangeable NPA codes"—area codes that did not have a 0 or 1 as the middle digit and could therefore be confused with the central office code—after January 1, 1995. Dialing 1 before the area code is most often required only for long-distance calls. Some telephone systems in early overlay areas do not accept a "1" before the area code for local calls; all Canadian landlines follow this pattern. However, in the three largest US markets (New York City, Los Angeles and Chicago), the initial "1" is required even for local calls for landline phones. All cellphones in North America ignore this and only require the user to dial ten digits. Ten-digit dialing in non-overlay areas A few areas that are not within an overlay plan nonetheless require ten-digit dialing if part of the local calling area is served by an overlay plan. One example of such an area is the Fort Knox Army base in Kentucky. The base itself is served by area code 502, which is not yet subject to an overlay plan, but its local calling area includes cities that are now served by the 270/364 overlay complex. As a result, Fort Knox imposed ten-digit dialing for all off-base numbers when the 270/364 overlay was established in 2014. A July 2020