irectory number of the required exchanges, or a combination. destination. For a local call, only the local number need be dialled. The subsequent introduction of subscriber trunk dialling (STD) For a trunk call, the number dialled is the subscriber's national for long-distance calls required the development of national num- number; this consists of an area code, followed by an exchange code bering plans (Barron, 1959; Francis 1959; Clarke, 1952; Nunn, and the number of the subscriber on that exchange. Subscribers 1952). Later, the introduction of international subscriber dialling must therefore obtain access to other services in the same way. (ISD) made it necessary for national numbering plans to conform to When services are provided locally (e.g. emergency services, an international numbering plan. operator assistance, etc.) the subscriber need only dial a local Numbering plans may be either 'open' or 'closed'. An open number. If a service is provided remotely, an area code is required. numbering plan has no fixed number of digits. Such a numbering In the BT network, these additional services are provided by the plan resulted from networks containing only step-by-step exchange- derived services network (Roberts, 1985; Roberts, 1987). This can S without registers; the number of digits to be dialled for each call be considered as an additional level in the trunk hierarchy. Although was determined by the number of switching stages it was routed service providers may share a common code of the same form as an through. A closed numbering plan has a fixed number of digits to be 'area' code, they may be anywhere in the country and the call is dialled for all calls, regardless of the geographical positions in the routed there via the derived services network. This caters for calls network of the calling and called subscribers. The need to use a where the call is charged to the called customer instead of the calling fixed number of digits arose from the introduction of register-con- customer (Freephone, 0800) or where t