A transaction method between a client and a terminal (2) connected to a telecommunication network comprises the transmission of at least one client identification (IDUI), a terminal identification (POSID), and transaction-specific data to a financial server connected to the telecommunication network.The terminal identification is read or entered in the terminal and transmitted by the telecommunication network to the financial server. The client is equipped with a SIM card which can be connected functionally to a mobile apparatus. The client identification transmitted to the financial server is read in the memory of the SIM card and transmitted to the financial server via at least one interface.