Catalyzes both phosphatidylcholine and phosphatidylethanolamine biosynthesis from CDP-choline and CDP- ethanolamine, respectively (PubMed:10191259, PubMed:10893425, PubMed:12216837, PubMed:37137909). Involved in protein-dependent process of phospholipid transport to distribute phosphatidyl choline to the lumenal surface (PubMed:10191259, PubMed:10893425, PubMed:12216837). Has a higher cholinephosphotransferase activity than ethanolaminephosphotransferase activity (PubMed:10191259, PubMed:12216837). {ECO:0000269|PubMed:10191259, ECO:0000269|PubMed:10893425, ECO:0000269|PubMed:12216837, ECO:0000269|PubMed:37137909}. This is the function of CEPT1 (choline/ethanolamine phosphotransferase 1, ENSG00000134255).