Probably involved in a salvage pathway for GDP-D-mannose biosynthesis (PubMed:25500577). Catalyzes the reversible phosphorolysis of 1,2-beta-oligomannan. In phosphorolytic reactions, prefers beta-1,2-mannobiose (beta-1,2-Man2) as substrate. Produces alpha-D-mannose 1-phosphate, which is the precursor of GDP-D-mannose (PubMed:25500577). beta-D-mannopyranosyl-(1->2)-D-mannopyranose + phosphate = alpha-D-mannose 1-phosphate + D-mannose kcat is 10 sec(-1) with D-mannose as substrate (for the synthetic reaction). kcat is 11 sec(-1) with D-fructose as substrate (for the synthetic reaction). kcat is 7.0 sec(-1) with beta-1,2-Man2 as substrate (for the synthetic reaction). kcat is 12 sec(-1) with alpha-D-mannose 1-phosphate as substrate (for the synthetic reaction). Optimum pH is 6.0 for phosphorolytic activity. Optimum pH is 5.5 for the synthetic reaction. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis. Monomer. Belongs to the glycosyl hydrolase 130 family.