The maximal mutual information (channel capacity) between the input and output of a telephone connection was calculated, taking into account both the frequency and time domain aspects of this problem. This leads to define an « information index » which can be computed from objective measurements. Several examples are given, both for digital coders and for analogue transmission: they show that the information index has a very good correlation with the results of opinion tests. This index therefore constitutes the basis of a mathematical model for predicting the transmission performance of speech in a telephone network.