One of the best ways to build customer loyalty is to recover excellently from our mistakes. Groove provides customer service software, and they've compiled a wonderful list of words that customers want to hear. I think these are phrases customers always want to hear, but they are most important when things are not going as the customer hoped. 

Groove: Word Choice Matters, 2014-Oct-21 by Len Markidan

1. I don't know, but I'll find out for you. 
2. I'd be frustrated, too. 
3. I'd be happy to help you with this. 
4. I'll send you an update by [day or time].
5. I really appreciate you letting us know. 
6. Is there anything else I can help you with?

I recommend clicking through to read the whole article, as well as the comments, which expand these ideas and especially recommend saying these words with sincerity, or not at all.