Cell C account

I had a Cell C contract 7 years ago. When my contract ended, I did not upgrade. 4 months later I found that they were still debiting my account. I phoned Cell C who told me I must send a letter to confirm that I am not upgrading as my contract does not end automatically. I did that and faxed them the letter. A few months later they were still deducting so I stopped the debit order. I phoned Cell c again and they told me the will fix it. <br> Now, 7 years later MBD call and sms me about the remainder of that contracts money. How can I pay for a upgrade that I never asked for or got the airtime or phone? I call MBD and explained it to them. They then wanted me to send them a copy of that letter. Where must I found the letter after 7 years and 2 moves? I will NOT pay that money.