incorrect billing

I did a contract with cell c over the phone , the sales I spoke to offered me a great deal that I cannot refuse, she offered me a 7inch Samsung tablet that I was going to pay R89.00 per month with no extra fee, I asked the lady if I will really pay that money she said yes because if cell does not debit 89.00 I must open a case for them because its against the law, to my surprise I was debited I was 4 or 5 hundred and something on the 1st month ,the following 629.00 I then call to find they said it will sorted, this month statement shows that Iam going to pay 1269.00 I called again they promised me that someone will call in 2 days nobody did I called again I was told that the call will be treated as urgent because I wanted to know the feedback before they deduct no one did up to today, im so fed up with cellc because they keep their promise and they fail to deliver, most of all they are not customer oriented, I will recommend it to anyone as their service is very bad they only milk people's money, I called 3 times this month but still no one bothers to call back and I want to cancel my contract with as I cannot 3000.00 in less than 3 months I took the contract, the hell with cell c