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In contrast to numbering plans, which determine telephone numbers assigned to subscriber stations, dialing plans establish the customer dialing procedures, i.e.the sequence of digits required to reach a destination.On the other hand, area codes that covered an entire state had the digit 0 in the middle.
In some areas, this led to early resistance to overlay codes, but this has usually been outweighed by the preference to avoid number changes for existing subscribers.
Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus.
Such systems may be supported by a private branch exchange (PBX), which controls internal communications between telephone extensions.
Since 1995, over 380 new area codes were added to the North American Numbering Plan.
Some areas used area code splits, by which an existing numbering plan area (NPA) was split into multiple divisions each assigned a new area code.Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements.A broad division is commonly recognized, distinguishing open numbering plans and closed numbering plans.Even in closed numbering plans, it is not always necessary to dial all digits of a number.