Probably part of the ABC transporter complex PhnD1C1E1. Binds strongly to inorganic phosphite and with very weak affinities to methylphosphonate (MPn) and phosphate. The complex may be composed of two ATP-binding proteins (PhnC1), two transmembrane proteins (PhnE1) and a solute-binding protein (PhnD1). Belongs to the phosphate/phosphite/phosphonate binding protein family.