Catalyzes the conversion of AMP and phosphate to adenine and ribose 1,5-bisphosphate (R15P). Exhibits phosphorylase activity toward CMP and UMP in addition to AMP. Functions in an archaeal AMP degradation pathway, together with R15P isomerase and RubisCO. AMP + phosphate = adenine + alpha-D-ribose 1,5-bisphosphate CMP + phosphate = alpha-D-ribose 1,5-bisphosphate + cytosine phosphate + UMP = alpha-D-ribose 1,5-bisphosphate + uracil Belongs to the thymidine/pyrimidine-nucleoside phosphorylase family. Type 2 subfamily.