s local-exchange number. change, a unique national number can be obtained by adding to this number a code identifying the exchange. These codes could be In addition to dividing the country into numbering areas (each allocated in a purely arbitrary manner. This has the advantage of with an area code) and determining a local numbering plan for each being able to accommodate the largest possible number of ex- area, it is necessary to determine appropriate dialling procedures for changes and makes it easy to cater for unseen developments. It has both trunk and local calls. The use of a single dialling procedure (i.e. the disadvantage that each originating trunk exchange must be able the full national number is dialled for both trunk and local calls) is to identify the code of every local exchange in the country to only justified if trunk traffic represents a high proportion of the total determine the routings and charges for calls. Instead, the country is traffic or if the country is sufficiently small for national numbers to usually divided into geographical areas, each having a separate code contain few digits. Otherwise, it is preferable to use two procedures, and containing a number of local exchanges. The originating trunk whereby local calls are obtained by dialling the local number and exchange then only requires to identify the area code to determine other calls by dialling the national number. The local exchange must the routing and charging rate for each call. be able to differentiate between them. This is usually done by means In general, a national number comprises three parts: of a trunk prefix digit. In the UK, this digit is '0'. Recognition of the 29/14 Telephone networks prefix causes the local exchange to route the call to the trunk a basic-access subscriber to have up to eight numbers in order to exchange. receive calls through the PSTN. An additional prefix can also be used to differentiate between In countries where the telecommunications service is 'lib- trunk calls and international calls. In th