Do not apologize first, you do not know what to apologize for until you have established a relationship with the customer and know the problem for which you are apologizing. Promising to do something completes the psychological phase. Customers relax in anticipation of their problem being resolved and become ready to help you to help them. Right now you need something from the complaining customer, the exact details of the problem and what they consider a satisfactory resolution. Make sure that your company's staff and your complaint handling system are prepared to respond to the gift that the customer is bringing.
Many strategies are available to determine what is necessary to satisfy the unhappy customer.
Fewer than two percent of complainers are trying to get something they know they do not deserve from the company. Most complainers don't expect the product or service to be provided free of charge, but they do expect to be recognized as someone who matters. Companies need not spend large amounts of money or give away the store to solve customer problems if the staff and store policy treat the complaint and the complainer with the proper respect. See if the customer has any further information for you. Make sure company systems, culture and training are designed to use the information received from complaints to prevent future problems.