Voice recording used for authentication is transmitted to the voice portal of the service provider not by way of the voice channel, but rather by way of a data channel. In this connection, the voice recording is sent not synchronous to speech and subject to loss, but rather asynchronously and loss-free, in an extra data package (for example advantageously as a Multimedia Messaging Service (MMS) data package). For this purpose, the resources that are available as standard items in most mobile phone terminals, such as digital voice recording and MMS transmission, can be utilized. Preferably, the subscriber's identification module or SIM card in the mobile phone terminal can store and implement the corresponding control program.