First, you need to allow them to vent their anger. If they are abusive, tell them you're willing to listen, but only if they stop using profanities. Next, hear what they have to say and note down key points. Third, re-state the main points so both of you are clear about the situation. Fourth, find a solution that fits within your company's policies and can redress the customer's problem. Fifth, confirm that the problem has been solved and that the customer is satisfied with the solution. You may not be able to do all of this in one phone call, but this is the essential process. The important part is to keep your cool at all times, and genuinely try to find a solution that will make the customer happy.