You do everything you can to avoid complaints but if you are faced with them, you deal with them constructively. After all, this can be an opportunity to make adjustments and improve performance yet further. You listen to what the client has to say and you formulate the right answer. You do all this with a feel for nuance, using positive language to deliver an optimum customer experience.
That can be a challenge even in your native language. But what if you have to do this in English?