A method and system for transmitting service data between telecommunication devices. Pre-paid access data including a first digital key and control data is stored in a memory module of a telecommunication device, a second digital key being stored on one or more control units of the telecommunication network. A validity criterion is determined based on the control data, and service data of the first telecommunication device is encoded by the first key as long as the validity criterion is fulfilled. The encoded service data is transmitted to the control unit, decoded by the second digital key, and transmitted to another telecommunication device.