Cell c told me I can't end my contract with them

I opened a samsung tablet contract with Cell c 2 months ago. They made a mistake by making my contract 12 months and not 24 months. I had to pay double registration money. I received the first registration money. The wrong branch code was inserted for the debit order to go off. I called them to say that i received a sms that said they haven't received my money for the first month. They said they would see what's wrong but couldn't find any problems. I received a few weeks ago the same sms again but just that I owe them R800 now and not R350 anymore because the debit haven't gone off for both months. I called them again. They said they don't understand why the debit order isn't going off they will try the universal code, but i owe R1200 now. Yesterday I received a sms saying that my contract has been suspended. I went to them yesterday after they end the call when i phoned them to end my contract because I'm not happy with the service. They didn't know how to end my contract. They told me head office is saying that I can only end it if I pay R5000 for the tablet. There is also something wrong with the tablets battery. I want to give everything back and end my contract now.