Answer: Follow up with a letter that documents the call and promotes goodwill.
Step-by-step explanation: The given case, belongs to the public relations concept under which the company tries to interact better with their customers, so that the customers enjoy a healthy experience and the image of the company remains positive.
The proper way to address a complaint call is to follow up by giving a letter stating the explanation apologies, excuses shall not be mentioned and assurance should be provided that such action would not be taken in future.