If I don't know an answer to a question when speaking with a customer, I just try to be as polite as possible. Let the customer know, hey, that is something that I'll need to look into further for you. If it's something they need to know right away, I have no problem putting them on hold and looking up that information or setting up a follow-up phone call, deal over with them at a later date if it's not something that needs to be answered right away.