The method needs a special processing equipment to couple the subscriber's telephone private branch exchange (PBX). For calling a subscriber of the extension by telephone, one should dial in two steps: first, to dial the telephone number of the subscriber's telephone PBX; second, after hearing a sound prompting to dial the number of the subscriber's extension telephone set, dialing the number. The adoption of the invention necessitates only to simply change the telephone PBX from manual switchover into automatic and needs no change of the equipment in urban telephone exchange. |