Abstract: According to the method of the invention two successive samples from each of two signal channels to be detected are alternately fed and not fed to the detection device. In consequence of this, fewer interference products are obtained, so that the processing time can remain relatively short. Moreover the two multi-frequency code samples (MFC-samples) of two channels can be processed simultaneously. A device (FIG. 3) examines on a time division basis the signalling time slots of two pulse code modulated frames of multi-channel signals (PCM-frames). Under the control of a microprocessor circuit (27) the pulse code modulated selected multi -frequency signal samples are processed in an input control circuit (28) when they are correlated with one another by being multiplied by sine and cosine reference series signals from a buffer (29). The correlation results are fed to accumulators (30, 33).