You can handle situations like yesterdays example gracefully and professionally every time with one easy sentence.

"Let me do a little research and call you back." The word research is very warm to most people because it has a very professional implication. The fact that you are willing to do "research" for them is usually quite gratifying. Just make sure to get as much information you can from them on the initial call and answer any questions that you are able to at that time.

To take it one step further, tell them when you will call back and call them earlier. Just and call them earlier. Just make sure you give yourself ample time to do your research and you will have under promised and over delivered making an excellent first impression!