I paid to cancel but you send me a bill!!!!

I had two contract phones with mtn. I was going to work overseas so cancelled my contracts, this cost me R6100. I paid this on the 8 may 2014 and once mtn had received the payment they cancelled my contract and made the phones pay as you go. Now in Feb 2015 i received an emailed account to say i still owe this money. I have proof of payment. I emailed back, but no response. I phoned and the guy who answered said he could not see what the problem is. He said his supervisor would phone me back, but no one phoned back. I have tried to phone again but i stay on hold for about 15 mins and no one answers! I emailed again. No response!!! I want mtn to contact me and sort this out as i have paid and now you are accusing me of not paying!!!