Bad service

I’ve had a bad experience with Mtn; I had a contract with them and my contract has expired and the acc is closed. Recently I received a call from their debt collectors Saya Recoveries; they say I’m owing them R2;126 because the contract was terminated before time. My problem is that the call came from them that I need to renew my contract of which I did not renew it. Mtn stopped deducting from my bank acc and they stopped loading data for me. I tried to communicate with mtn; but all my effort of getting help failed