Insincerity is the noun: the quality of faking feeling, or one stretch of it. “I could hear the insincerity in his voice” means the voice itself told you the kind words were not real. You can talk about someone’s insincerity as a habit, which is the usual way, or, more rarely, about an insincerity as one fake remark. It is a slightly formal word — a teacher, a review, a newspaper. A friend would more often say “he’s so fake.” Though actually, calling out insincerity is itself a strong move; people keep the word for when the gap between the nice words and the real feeling is obvious.