This contribution reports about a method to automatically detect the disturbing Robot Voice and Ping Pong effect which occur in GSM transmitted speech. Both effects are caused by the frame substitution technique, recommended by the GSM standard: in these cases the transmitted speechmay be modulated by a disturbing 50 Hz component. These modulations can be detected very easily in the frequency domain. By a framewise comparision of the modulation amplitude of an undisturbed clean speech signal with a test signal it is possible to locate the occurrence of Robot Voice and Ping Pong very precisely. Comparing human perception to the outcome of the proposed algorithm shows a high degree of correspondence.