Cell C can't allocate money

I decided in 2013 that I didn't want to upgrade my blackberry cellphone, but rather want to end my contract and switch over to pre-paid.<br> I went into The keywest Mall branch, and received a cancellation letter with a reference number, saying that I will not upgrade, but switch to pre-paid after my contract ended (which was in 2 months time).<br> Over that time my bank suggested that I must change my account from savings to cheque, so that meant I had to change my account nr as well. Since my cell c contract was paid by debit order, I told cell c that I would pay my last to payments straight into their bank account, which I did (and still have the proof of).<br> Now, more than 2 years later, I receive a sms from NuDebt Management, saying tha I still owe cell C money and that I will get blacklisted if I don't pay. I again want into the Keywest Mall Branch, told them the whole story, but they don't get back to me - that was 2 months ago. I gave my proof of payment and my Cancellation letter from cell C to NuDebt, but Cell C apparently don't know what happened, and never allocated my last 2 payments that was paid straight into their account.