An apparatus and method for determining a source of an unauthorized copy of speech signals in a conference call session. A conference bridge receives a speech signal during a conference call session with a plurality of end user devices attending. The conference bridge provides one or more dynamic, end user specific watermarks corresponding to the one or more end user devices. Each watermark is adjusted based on characteristics of the speech signal to make the watermark imperceptible to a human being. The speech signal is then embedded with the adjusted watermark in real time to generate a modified speech signal.