Poor accounting service by Vodacom inexplicable and frustrating to the bone***

I have an existing account/contract for 3 different devices with 3 different expiry dates for each of the devices. The one contract EXPIRES IN February 2020. I have been paying a specific amount on this device which amount is valid till the end of the contract. The past two months I have suddenly been charged with a totally different amount on this device and no-one knows why! Despite me questioning the account now Vodacom all of a sudden wants to terminate the service despite the fact that the alleged overdue (arrears) amount is entirely their fault. How does Vodacom explain their poor bookkeeping practices?!?!