Line prompting should always be discussed with the director. Some absolutely HATE when "LINE!" is called, and will get REALLY pee'd off if an actor "Disrupts their creative process" by calling "line". I had one director that wanted the cast to do a loud(ish) tap of their left foot if they needed prompting. It worked, although I could not usually tell if it was their left foot, any time I heard a loud tap, I would look up, confirm they were stalling and prompt.
The one thing I do hate. People who click or whistle to get your attention, either for a line or for other reasons.