MTN *********** and I must pay

I have had top up cellphone contracts with Nashua Mobile for the 11yrs and received excellent service from them. Since Nashua is no more, and moved us over to the Vodacom and MTN, things has just gone down hill and I am now fuming mad. I upgraded my son's contract last year October and he wanted an S3 mini, I dealt with Amanda van Wyk at NB at the time, she said top up is no more but they can make the my choice 50 minutes like a top up, so i pay one amount and top up with data. I received my bill where it clearly sets out how the contract is structured. When the contract was handed over to MTN clearly they did not check and see how this contract is structured and just opened up the contract which i only discovered when they deducted 7k off my account. I have been to MTN Century where i dealt with Shihaam Jackoet and having endless arguments that it's there stuff up not mine and i have gathered all my proof, I told them to request the voice recordings where the contract was agreed upon, to this date I'm still waiting,my e-mails are read but no response is received. I've handed over now for 15k, Think again MTN I'm not paying for your incompetence! someone pls advise what i can do