Do we know what kind of phone he had? Was it possibly water resistant or waterproof? Or if he had a case on it that was water resistant? That could explain why the phone was still operable while getting wet.
I followed this case early on and seriously thought there would have been an arrest by now as IMHO he did not accidentally drown.
I recall it was an android on TMobile but I am not sure if we ever found out the make and model. If he was being drowned during the voice mail I would guess his head was being held in, with the phone out of the water in his coat or pant pocket.