How to handle customer complaints:

Who is the Customer?
It is mandatory for any businessman to fully understand his customer because the purpose of any business is to create and keep a customer. Your customer is anyone who gets the result of your effort. These include both traditional clients(external)outside your organisation as well as colleagues at all levels.

To handle there complaints an ENTREPRENEUR should:

Listen without interrupting.
Show empathy.
Guide the conversation with open question.
Who/What/Where/When/Why/How?
Write down the information
Discuss the resolution
Agree on course of action
Act to resolve the complaint now: if possible (if not, set a time by which the problem will be resolved)
Ensure the action is carried out.
Keep the Customer informed of progress.