Catalyzes the first step in the synthesis of phosphocholine by converting phosphoethanolamine into phospho-monomethylethanolamine (N-methylethanolamine phosphate). 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. phosphoethanolamine + S-adenosyl-L-methionine = H(+) + N-methylethanolamine phosphate + S-adenosyl-L-homocysteine Feedback inhibition by phosphatidylcholine. Phospholipid metabolism; phosphatidylcholine biosynthesis; phosphocholine from phosphoethanolamine. Necessary at multiple stages in the worm's life cycle, its disruption alters worm growth and development. Belongs to the class I-like SAM-binding methyltransferase superfamily.