Randomly deciding I owe them R6000 when I am due to upgrade, accounts team are USELESS

For the past 7 months cell c has blocked my phone EVERY month no matter how many times I send them the proof of payment. I am due for an upgrade and cannot upgrade because suddenly I am over R6000.00 in arrears??? I have sent proof of payments for the whole year and no response??? This is absolutely ridiculous! I think it is time to move to a different provider as it seems you are trying to chase your customers away. PLEASE fix this issue TODAY still.