Abstract: The invention relates to a method that can be used in telecommunications networks, in particular comprising ISDN connections, to identify, retain and localize an interruption of speech/data channels. Currently, an impaired connection in the signaling channel cannot be differentiated from an unimpaired connection. The input of a tone which is continuously transmitted allows a pause in speech to be differentiated from an interruption of the speech/data channel from a measuring standpoint. It is therefore possible to measure and localize an interruption in an impaired connection. Alternatively, instead of creating a tone, a definable bit pattern can also be input into the B channel.