Cliente no cancela y no se va a presentar

I called the customer by phone and he told me that he does not think he could show up on the date of the reservation. But he does not cancel my reservation, I call him by phone and he does not answer. I want to release the days so that another person can reserve. I already spend 10 days for a reservation that was not presented or canceled. and I lost 6 more days for another one that I canceled the same day that had to be presented.