1. In a mobile telephone service network covering a substantial geographic surface, said mobile telephone service network including a plurality of mobile telephone exchanges and a national center, each of said plurality of mobile telephone exchanges being connected to a telephone system and controlling an assigned cell-structured mobile telephone area, and each of said plurality of mobile telephone exchanges being connected through a data transmission network to said national center; an improved mobile telephone location system comprising: means present at each of said plurality of mobile telephone exchanges for monitoring the presence of mobile telephone units within the cell-structured mobile telephone area assigned to that mobile telephone exchange; means present at each of said plurality of mobile telephone exchanges for transmitting a location request signal to said national center for a specified one of said mobile telephone units not present within the cell-structured mobile telephone area assigned to that mobile telephone exchange; means present at said national center for communicating mobile telephone unit location information to each of said plurality of mobile telephone exchanges, said means for communicating being responsive to said location request signal for a specified one of said mobile telephone units to communicate mobile telephone unit locator information for said specified one of said mobile telephone units; means present at each of said plurality of mobile telephone exchanges for signalling the presence of said specified one of said mobile telephone units within the cell-structure mobile telephone area assigned thereto when said specified one of said mobile telephone units is within the cell-structure mobile telephone area assigned thereto and said mobile telephone unit locator information for said one of said mobile telephone units has been received; and means present at each of said plurality of mobile telephone exchanges for receiving information defining the position of said specified one of said mobile telephone units as a result of a location request signal issued thereby and said means for signalling the presence and, said means for receiving acting to establish a communications channel to said specified one of said mobile telephone units.