Catalyzes the reversible phosphorolysis of 1,2-beta-oligomannan (PubMed:26632508). In phosphorolytic reactions, prefers beta-1,2-mannobiose (beta-1,2-Man2) as substrate, but can also use beta-1,2-mannotriose (PubMed:26632508). beta-D-mannopyranosyl-(1->2)-D-mannopyranose + phosphate = alpha-D-mannose 1-phosphate + D-mannose kcat is 31 sec(-1) with D-mannose as substrate (for the synthetic reaction). kcat is 3.9 sec(-1) with D-fructose as substrate (for the synthetic reaction). kcat is 12 sec(-1) with beta-1,2-Man2 as substrate (for the synthetic reaction). kcat is 33 sec(-1) with alpha-D-mannose 1-phosphate as substrate (for the synthetic reaction). kcat is 35 sec(-1) with beta-1,2-Man2 as substrate (for the phosphorolytic reaction). kcat is 12 sec(-1) with beta-1,2-mannotriose as substrate (for the phosphorolytic reaction). Optimum pH is 7.0 for phosphorolytic activity. Optimum pH is 6.0 for the synthetic reaction. Homodimer. Belongs to the glycosyl hydrolase 130 family.