If a customer gets angry over your business services, apologize quickly with this sample apology letter:
Hello Mr. Linus,
I'm sorry to hear that you are having trouble getting in touch with our customer service hotline. If you email your address and order number to email@example.com then we can have a look into your order. Thank you for letting us know about the matter of your shipment, we will get back to you as soonest possible.