from a major, international, electronic device corporation...
When someone tells you something, repeat back to them what they just said to you.
They will say about you, "this person really listened to me."
I know, I know, it sounds redundant. I personally do not need to hear a recap of what I just said to someone to make me believe that they listened to what I just said. However, there is a surprisingly large percentage of the population who need you to repeat back to them what they had just said before they acknowledge that you "listened to them".
Edit to add:
This is good advice for anyone who deals with people who just won't stop complaining no matter how much you help them. Oh yeah, that's another thing-- after you have repeated back what they said to you (their complaint or request or whatever) and you went ahead and did something, MAKE SURE TO TELL THEM what you did in order to solve their issue, possibly two times for good measure, and also repeat back to them the entire scenario about why they called to complain, what you did to solve their issue, and is there anything else? About 15% to 20% of the population needs that much verbal response from the listener before they believe that you actually listened to what they had to say.