Ten-digit Dialing - Overlay Plans

Overlay Plans

The introduction of overlay plans as a means to reduce the need for phone numbers to change as a result of adding new area codes meant that one geographic area could be associated with more than one area code. This is disadvantageous to new service providers as existing providers can issue numbers in the familiar area code. In order to promote fairness among carriers, 10-digit dialing became an FCC requirement; though the "1" before the area code is often required only for actual long distance calls. Some phone systems in early overlay plan areas still do not accept a "1" before the area code for non-long-distance calls.

The added dialing requirement, coupled with the need to remember which of the area's concurrent area codes applied to a given number, damaged the popularity of overlay plans, which themselves were introduced as a means to reduce the inconveniences associated with the traditional split plans.

As overlay plans have spread to more areas, 10-digit dialing in the U.S. and Canada is becoming increasingly common. However, 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 10-digit dialing. This requirement 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 1 Jan 1995.