Complex sentence: Wynschenk never talked about his past, until after he got a phone call in 1972 from a religious-school teacher, who had discovered that Wynschenk was a Holocaust survivor from his son, Mike.
Split into simpler sentences: Wynschenk never talked about his past, until after receiving a phone call in 1972 from a religious-school teacher. The teacher had discovered that Wynschenk was a Holocaust survivor from his son, Mike.