Catalyzes the conversion of adenosylcobalamin 5'-phosphate to adenosylcobalamin (vitamin B12); involved in the assembly of the nucleotide loop of cobalamin. Also catalyzes the hydrolysis of the phospho group from alpha-ribazole 5'-phosphate to form alpha-ribazole. adenosylcob(III)alamin 5'-phosphate + H2O = adenosylcob(III)alamin + phosphate alpha-ribazole 5'-phosphate + H2O = alpha-ribazole + phosphate Nucleoside biosynthesis; alpha-ribazole biosynthesis; alpha-ribazole from 5,6-dimethylbenzimidazole: step 2/2. Belongs to the phosphoglycerate mutase family. Extended N-terminus.