Unauthorized increase

There has been an increase in my subscription, which i am not happy about as it is not the amount that i agreed to when i took out a contract with Vodacom a year ago. I phoned the accounts department as the vat for my last two invoices are incorrect and the amount to be paid for the last three months all differ. The guy at the accounts department said that he can't do anything about it and that i should email my complaint to Vodacom, which i have done and no one has contacted me yet.

I have a closed line, therefore my charges are to be the same every month. On my invoice for April there was an increase on my line rental and my invoice for May shows an additional line rental increase.

Would appreciate it if someone could come back to me on this issue