Vodacom overcharges on existing contract

I started a new data contract at vodacom in May 2018 which I got ONLINE for R279 per month for 20gig + 20gig night owl. A few months into the contract my monthly bill changed to R299 for the same package without any reason or notice. I called vodacom on which I was firstly informed that the price increase was due to the VAT increase but since VAT changed to 15% before I got the contract, vodacom had to come up with another excuse since I proofed to them that it was NOT VAT related. First few times I called, the consultant did not want to give me a reference number and I demanded a reference number for the next query on which I got ref nr.*** with no resolve and then ref. nr.*** which vodacom later sent me a sms stating that the query was resolved but it was not. Next ref nr. *** but up till today no resolve. Vodacom just said that they have the right to change tariffs on which I hung up the phone since it was clear that vodacom expects from me the paying customer to just except whatever vodacom changes on my monthly bill, but I refuse to except it. I WANT A REFUND OF THE WRONG BILLING AMOUNT(R20 each month) FROM THE DATE IT WAS CHARGED RIGHT UP TO THE LAST INVOICE AND I WANT VODACOM TO EXCEPT RESPONSIBILITY FOR THEIR MISTAKE AND I WANT THEM TO APOLOGIZE!