As soon as this feature is activated a room can be monitored from any telephone configured in the same PBX. The feature is activated from the telephone in the room that is to be monitored. For activation a code with a maximum of 21 digits must be entered which may only be known to the activating person. Monitoring remains active until the handset of the telephone in the monitored room is placed on-hook or the loudspeaker is switched off. To activate the actual monitoring process the telephone in the monitored room is called, then the feature code and the secret code number can be entered. Only the receive channel is switched through for monitoring purposes.