RedactedNarrative: Vessel 1 Operator was drifting on Kirby Lake when his cellular telephone rang.  The Operator stated that when he stood up in the vessel to get his cellular telephone out of his pocket he lost his balance causing him to fall overboard.  The Operator stated that when he fell overboard, it caused the gunwale to drop below the surface of the water and the vessel immediately filled with water and sank.  The Operator stated that he was able to grab a PFD and swim to shore, but left the lake making no attempt to contact law enforcement to report the sunken vessel.