Separate your feelings from the situation.
Repeat the customer’s concerns. Once the customer is done venting, make sure you know exactly what it is they are upset about. If you are still feeling a little unclear, repeat what you think the customer is upset about, or ask him questions. Repeating the problem back to the customer will show him that you were listening, and will also let you confirm the problem that needs to be fixed. Let the customer know that you are genuinely sorry that this happened to them--regardless of whether or not you think they are being a bit dramatic about the situation. If you are in the process of handling a situation and the customer demands that you call your manager or supervisor over, it is best to follow the wishes of the customer.