Abstract: A communications system of a motor vehicle is disclosed in which a communications device (101) for setting up a mobile radio link is installed in the motor vehicle, which mobile radio link also has a short-range radio device. An external mobile radio device (118; 119) also additionally has such a short-range radio device. If the mobile radio device (118; 119) enters the radio range of the short-range radio device of the communications device (101) in the vehicle, data is exchanged between the devices over the short-range radio link. A comparator module in the communications device of the vehicle compares the received data with stored data with respect to specific features which contain an identifier of the mobile radio device (118; 119). If an authorized mobile radio device (118; 119) is detected, a call diversion is activated automatically within the cellular mobile phone network. Furthermore, a corresponding communications device (101) and a method for operating the communications system are proposed.