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 1,2-beta-oligomannan with a degree of polymerization (DP) of 3, 4 and 5. Produces alpha-D-mannose 1-phosphate, which is the precursor of GDP-D-mannose (PubMed:25500577). [(1->2)-beta-D-mannosyl](n) + phosphate = [(1->2)-beta-D-mannosyl](n-1) + alpha-D-mannose 1-phosphate kcat is 1.9 sec(-1) with D-mannose as substrate (for the synthetic reaction). kcat is 2.0 sec(-1) with D-fructose as substrate (for the synthetic reaction). kcat is 25 sec(-1) with beta-1,2-Man2 as substrate (for the synthetic reaction). kcat is 43 sec(-1) with beta-1,2-Man3 as substrate (for the synthetic reaction). kcat is 22 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 5.0 for the synthetic reaction. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis. Homodimer. Belongs to the glycosyl hydrolase 130 family.