Abstract: A system for monitoring of telephone calls on a plurality of inbound and outbound voice channels made to and originating from a common private branch exchange (PBX) to detect fraudulent activity. Monitors and detects audio data on two or more of the voice channels. Includes analysis of binary data streams on at least one inbound voice channel and at least one outbound voice channel and comparing said streams by a sliding window to slide a sample frame of one channel binary data stream backwards and/or forwards relative to the other binary data stream to synchronize the inbound voice channel and outbound voice channel. The comparison determines if a data match is present between the compared inbound channel and the outbound channel and blocks the at least one outbound voice channel, if a data match is found with at least one inbound voice channel.