Bad contract service

I upgraded my contract last year November ang received 2 phones, 1 of the phones was faulty however I was advised to return both. I returned both phones and cancelled the contract. However vodacom didn't cancel the contract and have been billing me since then. The account is now handed over and ruining my credit record. I have been calling non stop and talking to different agents who promise to call me back. Someone a manager please call me.