Subscription cancellation

My father in law cancelled his plan with the company, he had to pay a cancellation fee and settle the account which was all done on the 4th and 6th of December 2023. He sold the car hence these arrangements. How is he still being called by different people harassing him and demanding payment? No one is able to state how much he owes exactly, no one is willing to let speak to the manager and no one seems to know that this was cancelled/closed. How you conduct business makes no sense! Please work on your systems and stop harassing people who have been loyal customers for years and do not owe you.