A customer walked in to my store the other day asking what seemed to be pretty basic questions. I really didn't give any consideration to the fact that she was testing me.
Had the customer not told me about this incident, I would have never known. But to tell the truth I wouldn't have changed my approach. Knowing how important everyone is that comes through our door we have to balance taking care of the customer in-store and handling the occasional phone call. It's happened to you right? What do you say when the phone rings and you are working with someone already? Do you politely excuse yourself to answer the phone? Could you take down the caller's name and number in order to call them back?
However you handle it, both customers need to know they are extremely valuable and you are willing to help them in an orderly and timely way. The saying, "You don't get a second chance to make a first impression" rings true here.