The invention relates to a connection device (1) which is used to connect mobile radio terminals (51, 52, 53) to an electronic system (2) arranged in a vehicle, and a base part (3) and an adapter part for said type of connection system. The connection system (1) comprises a base part (3) which is permanently fixed in the vehicle and one or several retaining parts (41, 42) for respectively receiving a mobile radio terminal (52, 52). The base part (3) comprises a first electric interface (11) which is connected to a signal processing device (2) arranged in the vehicle, said device performing at least partial functions of a handsfree device. The base part (3) also comprises a second mechanical and electrical interface (12) which is used to connect retaining parts (41, 42) in order to receive mobile radio terminals (51, 52), in addition to a communication device (5) for wireless communication with a mobile radio terminal (53) via a third interface (13). The communication device (5) is configured in such a manner that it communicates with the signal processing device (2) via a first interface (11) by means of a first protocol and it converts the first protocol into a third protocol in order to communicate with the mobile radio terminal (53) via a third interface (13).