For this, I am quite honest with either the customer, the client, or even co-workers and let them know that I don't know the answer to their question. However, I will confirm that I have their question correct and follow up on the information, whether that is doing research of my own or asking somebody more knowledgeable, and then I will follow up with whomever asked in their preferred method of communication, whether that be phone, email, in person, or otherwise.