The Gemoro and Rashi say that the shvotim said directly to their father: Shma Yisroel... (listen Yisroel,...).
Isn't it forbidden for a child to directly address his father by his name?
Shalom Rav,
You are asking a very good question; your question must be addressed by Mefarshim, but I could not find it in the local commentaries. I suggest that Yakov's sons, sensing what was on their father's mind, cited the verse in the Torah (which presumably they knew) and, due to the circumstances in which they quoted it and based on its dual implications, Yakov would have picked up the message. They were quoting the verse and not addressing him by name. Yakov understood it as the answer to his quandry.
B'Virchas Kol Tuv,
Eliezer Chrysler
The Sefer Lechem Yehudah (Hilchos Keri'as Shema 1:4) asks your question. He answers that the sons turned their father with titles of honor, such as "Adoneinu Avinu Yisrael." In this manner there is no prohibition against calling one's father by his name.
The Shelah ha'Kodesh (Derech Chayim Tochechos Musar #3) gives a similar answer and says that the name "Yisrael" itself includes titles of honor, as the name itself means greatness and authority, and thus it is as if they called him "Adoneinu."
Y. Shaw