Catalyzes the hydrolysis of inorganic orthophosphate from phytate. Only phytate, ADP, and ATP were hydrolyzed (100, 75, and 50% of the relative activity, respectively). 1D-myo-inositol hexakisphosphate + H2O = 1D-myo-inositol 1,2,4,5,6-pentakisphosphate + phosphate Ca(2+) is required for its activity and/or stability. Optimum pH is 7. Optimum temperature is 55 degrees Celsius. By phytate.