he caller is charged at a trolled switching systems. The registers had a limited digit-storage premium rate for the call (e.g. message services). capacity and relied on this being full to indicate that sufficient In countries where the telecommunications service is 'lib- information had been received to enable a call to be set up and the eralised', there are several competing PTOs; thus, a subscriber on register released. Modern switching systems can cater for numbers one network must be able to obtain access to subscribers on other of varying length. Many national numbering schemes therefore networks. In the UK, other PTOs have 'area' codes and BT provides contain numbers having several different lengths, up to a given access to them via this derived services network. For example, calls maximum number of digits. are routed to the two competing cellular radio mobile systems There must always be an upper limit to the maximum number of (Cellnet and Vodafone). digits in a national number to comply with international require- In the case of a call to a mobile-radio subscriber, the number ments. The CCITT has recommended (CCITT, 1989a) that the dialled represents the identity of the subscriber rather than his maximum number of digits for an international call shall be 11. The location (which may be anywhere in the country). Ideally, this maximum number of digits in a national number is thus 11-N, where Numbering and charging 29/13 NETWORK DATABASE ACCESS TO/FROM ALL DDSN SWITCHES DIGITAL DERIVED SERVICES NETWORK DDSSC DDSSC DDSSC DDSSC (Only 4 centres shown for clarity) EXISTING PUBLIC SWITCHED TELEPHONE NETWORK AMSU: Analogue main switching unit Analogue local exchange DDSSC: Digital derived services switching centre Digital local exchange DMSU: Digital main switching unit Service provider Figure 29.13 Derived services network N is the number of digits of the country's code in the world 1. An area code. numbering plan. 2. An exchange code. Since each subscriber has a directory number in his own ex- 3. The subscriber'