Abstract: Telecommunications system including a fixed tele- and datacommunications system, hereinafter called a fixed system, and a radio communications system, for instance a GSM-system, hereinafter called a mobile system, the mobile system including mobile units. A respective mobile unit of a number of mobile units is arranged for connection to an optional subscriber line or extension line in the fixed system via a connection device (for instance a jack), and the fixed and/or the mobile system is/are arranged with identification devices which identify via said connection device, to the fixed system, a connected mobile unit for enabling billing for the use of the mobile unit in the fixed system.