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. N-methylethanolamine phosphate + S-adenosyl-L-methionine = H(+) + N,N-dimethylethanolamine phosphate + S-adenosyl-L-homocysteine N,N-dimethylethanolamine phosphate + S-adenosyl-L-methionine = H(+) + phosphocholine + S-adenosyl-L-homocysteine Feedback inhibition by phosphatidylcholine and also by S-adenosylhomocysteine. Phospholipid metabolism; phosphatidylcholine biosynthesis; phosphocholine from phosphoethanolamine. Essential in multiple developmental steps, its disruption causes developmentally impaired worms. Belongs to the class I-like SAM-binding methyltransferase superfamily.