<P> The added dialing requirement, coupled with the need to remember which of the area's coincident area codes applied to a seven - digit local number, damaged the popularity of overlay plans, which themselves were introduced as a means to reduce the inconveniences associated with the traditional split plans . </P> <P> As overlay plans have spread to more areas, 10 - digit dialing in the U.S. and Canada is becoming increasingly common . However, most areas not within an overlay plan can still use 7 - digit dialing for local calls, although long distance calls within the area code may have required ten or eleven digits . 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 . </P> <P> A few areas that are not within an overlay plan nonetheless require 10 - 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 10 - digit dialing for all off - base numbers when the 270 / 364 overlay was established in 2014 . </P>

When did they add area codes to phone numbers