Bad Customer Experience

I booked a car but wanted to extend for a few days. I called them and they said it was fine and I will just have to pay for the extra days. However, I then began receiving calls at odd hours like early in the morning even on Sunday asking me how long am I am going to keep the car for. Then, I received a voice mail saying that my payment is outstanding? When I raised it with them, they simply insisted that this is their policies and I was accused of showing disregard to their policies. When I tried to contact their customer care officer, whose name is mentioned in their after service message, he refused to take my call.