Specifically cleaves both streptomycin-6-phosphate and, more slowly, streptomycin-3''-phosphate during the biosynthesis of streptomycin. H2O + streptomycin 6-phosphate = phosphate + streptomycin Binds 1 Mg(2+) ion. Binds 2 Zn(2+) ions. Antibiotic biosynthesis; streptomycin biosynthesis. Belongs to the alkaline phosphatase family.