unfair billing

I took a contract with cell c this year March, Samsung J5 and I was paying R229 from April. and in August they debited R439 from my account and I went to the cell c store in newtown and they said on their side its not showing anything so they don't know why I was debited that amount. they phoned the contact centre and contact centre said its because I didn't pay my account in July and I must send proof that they debited in July. I went to my bank and printed the statement and went back to the store (newtown), they faxed my statement and called again and now because they have proof that I paid they changed the story. they said my contract is uncapped and I asked how and when because since I had the contract I was paying R229 and my limit was R50. they transferred my call to another department because they said they don't know what happened and they transferred me to another department and they said the same thing. I was at the newtown branch for about 1hour 30 minutes and did not get any help. and September again statement was saying R439 and I phoned the customer care to find out what happened and was on the call for about 30 being transferred from one department to another and did not get the answers. even today I don't have answers and I tried to find out who changed my contract or what happened. I am not going to pay the R439 because that suppose to be amount im must pay for 2 months.