Incorrectly billed

Hi, I have a contract with cell c, I called in in February to cancel the contract because I don’t need the data any longer. I was advised to pay a cancellation fee of R5000 to cancel because my contract only ends in August, I was told to call in July to cancel. July I called in and cancelled my contract and an sms was sent and I was told my last instalment will be end of August. I received my invoice this morning and I’m being billed R1557.06 instead of R598.90. I called in and the call centre says My contract is ending on the 4th of August and I had to call in before the 4th of July. No one told me this when I called in for the cancellation. I’m extremely dissatisfied