Acknowledging a complaint
The worse thing than having a customer complain is doing nothing about it. Sure, there may be times when nothing much can be done, but that is no excuse to ignore a complainant. The least a company can do is acknowledge and empathise. In many cases this in itself calms the irate customer.
A common practice during times when service personnel are faced with a fast deteriorating situation involing irate customers is to abandon post and flee. There are even times when paramilitary forces have been called in to control angry customers. All of which could have been avoided if in the first place the service providers had acknowledged and responded to customer complaints.