Very poor client service

Good Day i have paid my account at mtn's legal department i November. Sins november i paid again till january at mtn as i was told that i still need to pay my account. I recieved a statment from mtn and it shows a negitive balance so in my eyes it says that mtn is owing me the balance. Phone the call centre with no luck i still dont have an awnser and on the credit beuers mtn state that they written it off. But legal i have paid my account up todate and i have a paid up letter from there legal department.