Catalyzes N-methylation of phosphoethanolamine, phosphomonomethylethanolamine and phosphodimethylethanolamine, the three methylation steps required to convert phosphoethanolamine to phosphocholine. Mediates a key step in the biosynthesis of choline, a precursor of the osmoprotectant glycine betaine. Has no ethanolamine- or phosphatidylethanolamine-N-methyltransferase activity. phosphoethanolamine + S-adenosyl-L-methionine = H(+) + N-methylethanolamine phosphate + S-adenosyl-L-homocysteine Inhibited by phosphocholine but not by choline, glycine betaine, monomethylethanolamine or dimethylethanolamine. Optimum pH is 7.8-8.5. Phospholipid metabolism; phosphatidylcholine biosynthesis; phosphocholine from phosphoethanolamine: step 1/1. Monomer. By salt stress. Belongs to the class I-like SAM-binding methyltransferase superfamily. PEAMT family.