e UK, the international prefix 'eralised', codes must be reserved to enable subscribers to obtain is '010'. The first digit (0) routes the call to the trunk exchange. The access to different competing networks. In the USA, different trunk next two digits (10) cause the call to be switched to the international prefixes are used to enable subscribers to choose different carriers gateway exchange. This three-digit prefix is followed by the inter- for their long-distance calls. In the UK, area codes may not use the national code for the required country and the national number of initial digit '8', since this provides access to the BT derived services the called subscriber in that country. network (Roberts, 1985; Roberts, 1987). This network caters both The UK numbering plan is illustrated in Table 29.1. Six of the for charging the call to the called subscriber instead of the caller largest cities are director areas. These have a uniform 7-digit num- (Freephone service, 0800) and for charging the caller at a premium bering scheme, consisting of a 3-digit exchange code and 4- digit rate (e.g. message services). Access is also provided to other net- subscriber's number. These areas are identified by 2-digit area works, e.g. the cellular radio networks of Cellnet and Vodafone. codes. Other areas have 3-digit codes and have numbering schemes Thus, '0836' might appear to be an area code, but '0836XXXXXX' with uniform 6-digit numbers or mixed schemes not exceeding six would be the number of a cellular telephone which could be any- where in the UK. Table 29.1 Typical UK national numbers The introduction of mobile services had led to numbers being associated with individual subscribers, rather than geographical locations as in the fixed PSTN. Ideally, this would apply to every- Type of area Prefix Exchang Subscrib Total body. There would be no need for abortive calls when subscribers e code number are away and no need for subscribers to have their numbers changed number if they move to another town. This would require every 