Billed on a concluded contract

I have been a good client to for the past 4 years with them till this past month August when I wanted to upgrade my cellphone contracts. Thus when i was told i am owing them +/- R2000. Before that i had called MTN to find out how much was i owing them for all my contracts with them because i knew August was the last month so i wanted to settle my accounts. And i was told you are only owing R96. Then in August my cellphone broke so i needed a cellphone when i got to the MTN shop at parkstation, I was told you are owing R96 so pay that so as for them to give me the 2 cellphones i needed. From the bank back into the shop thus when i got the shock of my day. We can not give you the phones because you owe us R2000 plus. Then I asked from where they rep told me from extra data. Of which my contract was that no extra data should be allowed if finished i will topup prepaid. Furthermore i was getting 2 gigabytes every month which i was not even finishing. Every month i would have excess +1500gigs. so this was a surprise. They called their head office to find out but they just said extra data, when i asked to speak to them, i was told use your fone to do that. i wrote to them but no goodansw