Well, that's a more straight forward, accept that you don't know the answer. Explain to the caller, person that you don't know the answer, and you'll get the answer for them. Make a note, and do some diligent, or make diligent effort to get the answer. And call back the person and give them the explanation, as best you can. But you have to admit that you don't know the answer to that question, and you will find out, you will make effort, you will get the answer, and you will call back the person and let the person know.