Embodiments of the invention include a system and a method for monitoring telephone calls on a plurality of inbound and outbound voice channels made to and originating from a private branch exchange (PBX) network to detect fraudulent activity. Audio data on two or more of the voice channels is monitored and detected. The detection comprises processing binary data streams on at least one inbound voice channel and at least one outbound voice channel, and modifying the bit sequence of the binary data stream of the at least one inbound voice channel by introducing a watermark therein. The streams are compared for determining whether the watermark is present in at least one outbound channel. If the watermark is found, the at least one outbound voice channel is blocked.