TheInternational Telecommunication Union(ITU) has established a comprehensive numbering plan, designatedE.164, for uniform interoperability of the networks of its member state or regional administrations. It is an open numbering plan, however, imposing a maximum length of 15 digits to telephone numbers. The standard defines acountry calling code(country code) for each state or region which is prefixed to each national numbering plan telephone number for international destination routing.Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus. Such systems may be supported by aprivate branch exchange(PBX) which controls internal communications between telephone extensions.In contrast to numbering plans, which determine telephone numbers assigned to subscriber stations, adial planestablishes the customer dialing procedures, i.e. the sequence of digits users are required to dial to reach a destination. Even in closed numbering plans, it is not always