Catalyzes the last two methylation reactions in the synthesis of phosphocholine, by converting phospho-monomethylethanolamine (N-methylethanolamine phosphate) into phospho-dimethylethanolamine (N,N-dimethylethanolamine phosphate) and the latter into phosphocholine. Phosphocholine is a precursor for phosphatidylcholine, a major component in membranes and a precursor itself in the production of glycoconjugates secreted by parasitic nematodes to avoid host immune responses