A device or ''dongle'' (30) for controlling communications between a Subscriber Identity Module (or SIM) (12), such as of the type used in a GSM cellular telephone system, and a computer, such as a Windows-based PC (10). The SIM (12) can be authenticated by the telephone network, in the same way as for authenticating SIMs of telephone handset users in the network, and can in this way authenticatethe user of the PC (10) or the PC (10) itself. Such authentication can, for example, permit use of the PC (10) for a time-limited session in relation to a particular application which is released to the PC (10) after the authentication is satisfactorily completed. The application may be released to the PC (10) by a third party after and in response to the satisfactory completion of the authentication process. A charge for the session can be debited to the user by the telecommunications network and then passed on to the third party. The dongle (30) provides additional security for the authentication data stored on the SIM by requiring a PIN to be entered and/or by only being responsive to requests received from the PC (10) which are encrypted using a key, which requests are generated by a special PC interface driver (38).