Catalyzes the hydrolysis of phytate (or myo-inositol hexakisphosphate, an indigestible organic form of phosphorus that is found in many plant tissues) to myo-inositol and inorganic phosphate. Dephosphorylates phytate in a stereospecific way by sequential removal of phosphate groups to produce myo-inositol 2-monophosphate. Also shows phosphoanhydride phosphatase activity and hydrolyzes the distal phosphoryl residues of GTP, the 5'-beta-phosphoryl residue of the regulatory nucleotide ppGpp and tripolyphosphates. Does not split most phosphomonoesters with the exception of the synthetic substrate p-nitrophenyl phosphate (pNPP), 2,3-bisphosphoglycerate and fructose 1,6-bisphosphate