When I don't know the answer to a question I would ask the person to wait while I double check and see if I can find the best answer or the correct answer from a colleague. I would ask the person if they don't mind waiting and if they don't mind waiting I would go and find out if it's going to take a little bit longer to find the correct answer for them. Then I would ask them if they want me to give them a call back or come back to them, contact them once I have the correct answer for them.