Usually, when my partner calls a T, I know the reason. However, there are those times when I'm down court or something similar and I don't know the reason. When that happens, most of the time I will just ask my partner "What happened?" After he tells me, I just nod and say "Oh, OK".
Whenever my partner asks me why I called a T, I usually say, "For the same reason I called the other seven so far this quarter."
