Unsatisfied answer

I have two contracts with MTN which is due for an upgrade in the near future, I have however decided that due to my financial position not to renew or upgrade my contract once it has finished. I phone the customer service department and they informed me that if I go to pay as you go after the expiry, I will lose all my airtime that I paid for every month. I have done my part of the contracts by paying my monthly fee, and now MTN do not do their part of the contract by taking away my airtime which i have paid for, in my eyes this is contract brake from MTN's side. You can't ***** something from me which I have legally paid for. This is not acceptable and as I have tried to sort this out or get a proper answer from Customer Service, they just say that it is the way it works. I also tried to change my email adres for my statements but without any success