Vodacom fibre deal

I have a contract with Vodacom for fibre and from Dec I have been billed incorrect amount. I have called them and they first told me about me being prorated as Vuma has changed the lines. I let slide and now they are continuing billing me more that our contract with no resolution back and forth calls. I am so ****ed with this and they don’t understand the inconvenience they are causing me while they have been investigating