When a customer complains to you, the very least you should do is acknowledge the complaint and set their expectations as to when you will reply. If the complaint is about a critical service i.e. telephone, credit card, etc. then your company should ensure that the customer who complains does not get penalised by your company's inability to respond quickly enough. So for example, if a customer complains they have been overcharged for telephone calls and you threaten to cut them off within say 7 days, don't take any action until you have reviewed the complaint fully even if it takes you say 21 days.