Major sphingomyelin synthase at the Golgi apparatus (PubMed:14685263, PubMed:17449912). Catalyzes the reversible transfer of phosphocholine moiety in sphingomyelin biosynthesis: in the forward reaction transfers phosphocholine head group of phosphatidylcholine (PC) on to ceramide (CER) to form ceramide phosphocholine (sphingomyelin, SM) and diacylglycerol (DAG) as by-product, and in the reverse reaction transfers phosphocholine from SM to DAG to form PC and CER. The direction of the reaction depends on the levels of CER and DAG in Golgi membranes (PubMed:14685263, PubMed:14976195, PubMed:17449912, PubMed:17982138, PubMed:19454763). Converts the newly synthesized CER, that is transported from the endoplasmic reticulum to the trans-Golgi by the Cer transport protein (CERT), to SM (PubMed:30242129). Can form a heteromeric complex with glucosylceramide synthase (GCS) increasing SMS activity and reducing glucosylceramide synthesis, a critical mechanism that controls the metabolic fate of CER in the Golgi (PubMed:30242129). Does not use free phosphorylcholine or CDP-choline as donor (PubMed:14685263, PubMed:14976195). Can also transfer phosphoethanolamine head group of phosphatidylethanolamine (PE) on to CER to form ceramide phosphoethanolamine (CPE) (By similarity). Regulates receptor-mediated signal transduction via mitogenic DAG and proapoptotic CER, as well as via SM, a structural component of membrane rafts that serve as platforms for signal transduction and protein sorting (PubMed:14976195, PubMed:17449912, PubMed:17982138). Plays a role in secretory transport via regulation of DAG pool at the Golgi apparatus and its downstream effects on PRKD1 (PubMed:18370930, PubMed:21980337). {ECO:0000250|UniProtKB:Q8VCQ6, ECO:0000269|PubMed:14685263, ECO:0000269|PubMed:14976195, ECO:0000269|PubMed:17449912, ECO:0000269|PubMed:17982138, ECO:0000269|PubMed:18370930, ECO:0000269|PubMed:19454763, ECO:0000269|PubMed:21980337, ECO:0000269|PubMed:30242129}. This is the function of SGMS1 (sphingomyelin synthase 1, ENSG00000198964).