A programmed BasicCard is used as the starting point in this article. We show how this ‘SIM card emulator’ can be adapted to test the most important functions that are available from the normal menu and the service menu of certain GSM phones.The SIM (Subscriber Identification Module) card is the indispensable key for the use of every GSM phone. It contains both secret codes as well as personal details of the user and is supplied to the customer by the network operator, who usually retains the ownership rights.A Smartcard using an open Operating System, such as the BasicCard, can be modified into an experimental SIM card, which allows a large number of experiments to be done independently from the network operators.