MTN the worst cellphone provider

I was contacted by one of MTN's distributors (BuyMondo) 2 years ago for a cellphone contract. Upon receipt, it was not the correct phone and it was sent back. Still, I was billed monthly. Eventually I was told via email that the contract has been stopped and I stopped the debit order by the bank. I never heard anything from MTN/ buymondo again including no emails, calls or sms's. Now, 2 years later, I am called by MTN's lawyers (VvM)regarding this unpaid account which I never used. I submitted all of my documents (10 to be exact) of proof including an affidavit, proof of collection etc, but still they respond saying that I have insufficient evidence. What more could you possibly need? I have never been so upset or frustrated with a cellphone company and have never in my life had bad debt. Now you have messed up my credit score. I'm so frustrated and angry and would not advice anyone to have a contract with MTN and especially not via BuyMondo.