A speech-based system for automatically detecting synthesized or natural speech, comprising - a telecommunication network, - at least one communication apparatus connected to the telecommunication network and configured to receive speech signals from a further communication device, the speech signals being transmitted over the telecommunication network, - a computational system comprising the following features: - a memory configured to store a classification algorithm, which has been trained by using a plurality of first training sets of data, each first training set of data representing a portion of a synthesized speech signal, each portion having a predetermined duration, and a plurality of second training sets of data, each second training set of data representing a portion of a natural speech signal, each portion having a predetermined duration, wherein the classification algorithm evaluates a first predefined index value, whenever one of the plurality of first training sets of data is used thereby indicating a synthesized speech, and wherein the classification algorithm evaluates a second predefined index value whenever one of the plurality of second training sets of data is used thereby indicating a natural speech, - a communication interface configured to receive speech signals addressed to the at least one communication apparatus, - a data processing unit configured to i) divide up the speech signals received at the communication interface into portions, each portion having a predetermined duration, ii) to evaluate an index value by executing the classification algorithm stored in the memory on at least one of the portions, and iii) to determine in dependence of the first and second predefined index value and in dependence of the index value evaluated, whether the at least one portion belongs to a synthesized speech signal or to a natural speech signal. - a telecommunication network, - at least one communication apparatus connected to the telecommunication network and configured to receive speech signals from a further communication device, the speech signals being transmitted over the telecommunication network, - a computational system comprising the following features: - a memory configured to store a classification algorithm, which has been trained by using a plurality of first training sets of data, each first training set of data representing a portion of a synthesized speech signal, each portion having a predetermined duration, and a plurality of second training sets of data, each second training set of data representing a portion of a natural speech signal, each portion having a predetermined duration, wherein the classification algorithm evaluates a first predefined index value, whenever one of the plurality of first training sets of data is used thereby indicating a synthesized speech, and wherein the classification algorithm evaluates a second predefined index value whenever one of the plurality of second training sets of data is used thereby indicating a natural speech, - a communication interface configured to receive speech signals addressed to the at least one communication apparatus, - a data processing unit configured to i) divide up the speech signals received at the communication interface into portions, each portion having a predetermined duration, ii) to evaluate an index value by executing the classification algorithm stored in the memory on at least one of the portions, and iii) to determine in dependence of the first and second predefined index value and in dependence of the index value evaluated, whether the at least one portion belongs to a synthesized speech signal or to a natural speech signal. - a memory configured to store a classification algorithm, which has been trained by using a plurality of first training sets of data, each first training set of data representing a portion of a synthesized speech signal, each portion having a predetermined duration, and a plurality of second training sets of data, each second training set of data representing a portion of a natural speech signal, each portion having a predetermined duration, wherein the classification algorithm evaluates a first predefined index value, whenever one of the plurality of first training sets of data is used thereby indicating a synthesized speech, and wherein the classification algorithm evaluates a second predefined index value whenever one of the plurality of second training sets of data is used thereby indicating a natural speech, - a communication interface configured to receive speech signals addressed to the at least one communication apparatus, - a data processing unit configured to i) divide up the speech signals received at the communication interface into portions, each portion having a predetermined duration, ii) to evaluate an index value by executing the classification algorithm stored in the memory on at least one of the portions, and iii) to determine in dependence of the first and second predefined index value and in dependence of the index value evaluated, whether the at least one portion belongs to a synthesized speech signal or to a natural speech signal. i) divide up the speech signals received at the communication interface into portions, each portion having a predetermined duration, ii) to evaluate an index value by executing the classification algorithm stored in the memory on at least one of the portions, and iii) to determine in dependence of the first and second predefined index value and in dependence of the index value evaluated, whether the at least one portion belongs to a synthesized speech signal or to a natural speech signal.