Displays phosphatase activity for serine/threonine residues. Has apparently no phosphoglycerate mutase activity. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Interacts with skn-1 isoforms a and c. Belongs to the phosphoglycerate mutase family. BPG-dependent PGAM subfamily.