My assistant uses phrases like "hold on a sec" or "this will take a moment", when it is about to call a function. This is not very useful for QnA assisstants. Prompting didn't help much. Is there another way to turn it off?

It is an inbuilt feature from Vapi. You can’t fix it via prompting. Also, as per what I know you cannot turn it off. So, why don’t you ask this question during daily office hours.