Catalyzes the phosphorolytic breakdown of the N-glycosidic bond in the beta-(deoxy)ribonucleoside molecules, with the formation of the corresponding free purine bases and pentose-1-phosphate (By similarity). Preferentially acts on 6-oxopurine nucleosides including inosine and guanosine (By similarity). inosine + phosphate = alpha-D-ribose 1-phosphate + hypoxanthine guanosine + phosphate = alpha-D-ribose 1-phosphate + guanine 2'-deoxyguanosine + phosphate = 2-deoxy-alpha-D-ribose 1-phosphate + guanine 2'-deoxyinosine + phosphate = 2-deoxy-alpha-D-ribose 1-phosphate + hypoxanthine Purine metabolism; purine nucleoside salvage. Homotrimer. Belongs to the PNP/MTAP phosphorylase family.