Involved in the N-glycan catabolism by catalyzing the reversible phosphorolysis of beta-1,4-D-mannosyl-N-acetyl-D-glucosamine (ManGlcNAc) in a sequential mechanism to yield N-acetyl-D-glucosamine (GlcNAc) and alpha-D-mannose 1-phosphate. It can also use beta-1,4-D-mannosyl-N-acetyl-D-glucosamine-N,N-diacetylchitobiose (ManGlcNAc2) and N,N-diacetylchitobiose (GlcNAc2) as substrates. 4-O-beta-D-mannopyranosyl-N-acetyl-D-glucosamine + phosphate = alpha-D-mannose 1-phosphate + N-acetyl-D-glucosamine kcat is 37 sec(-1) for phosphorylase activity with GlcNAc (at pH 5.5 and 30 degrees Celsius). kcat is 22 sec(-1) for phosphorylase activity with GlcNAc2 (at pH 5.5 and 30 degrees Celsius). kcat is 210 sec(-1) for phosphorylase activity with ManGlcNAc (at pH 5.5 and 30 degrees Celsius). Optimum pH is 5.5. Homotetramer. Belongs to the glycosyl hydrolase 130 family.