National regulatory authorities shall encourage the earliest possible introduction of operator number portability whereby subscribers who so request can retain their number(s ) on the fixed public telephone network and the integrated services digital network ( ISDN ) independent of the organisation providing service , in the case of geographic numbers at a specific location and in the case of other than geographic numbers at any location , and shall ensure that this facility is available by 1 January 2000 at the latest or , in those countries which have been granted an additional transition period , as soon as possible after , but .