WRONG BILLING /REFUND DUE

I renewed my contract after two years of paying R 500,the new contract I was suppose to pay R 170 .But every month Telkom send me an invoice of the old contract,and they debited R 500 .I need to call them every month to rectify this but they don't bother to call me back or refund me my money .If there was a way to cancel this contract I would because i'm tired of the way they do business they could care less about their loyal clients and customer service its been two weeks of me waiting for a response from this company and no one ever calls you but they don't forget to debit your account ,I'm furious