Glycosyltransferase that transfers an alpha-D-mannosyl residue from GDP-mannose into lipid-linked oligosaccharide, forming an alpha-(1->2)-D-mannosyl-D-mannose linkage. Required for addition of mannosylphosphate in yeast mannan. Recognizes any oligosaccharides with at least one alpha-1,2-linked mannobiose unit. Protein modification; protein glycosylation. Present with 4380 molecules/cell in log phase SD medium. Belongs to the glycosyltransferase 15 family.