Using density functional theory (DFT) calculations, we investigate the specie obtained from the stepwise on-cage P-doping of the silicon fullerene Si20H20. It is found that P-doping decreases the HOMO-LUMO energy gap of the fullerene and could be used to tailor its optical properties. Pairing of P atoms within the fullerene's cage is not favored owing to the repulsion between the phosphorous lone-pairs and the preference for Si-P bond formation. Replacing all the SiH moieties with P affords the dodecahedral P20 fullerene. The fully oxidized P20O20 species is characterized by exceptionally high ionization potential (10.73 eV) and large electron affinity (6.71 eV). {\textcopyright} 2009 Elsevier B.V. All rights reserved.