Unfair billing,, Vodacom is a ****

I took a phone on contract with vodacom on the 3rd of January 2023...I missed a payment on that month but the following month I paid all the money that they billed me.. I thought since I paid all the money plus the arears, the following month I'm going to pay the amount we agreed on initially but which it was R450 per month. To my surprise I was billed R750 and I paid it hoping it's gonna be back to normal the following month but to my big surprise I was billed R1000 rand, that's where I called them to enquire, they told me about arears but I was surprised cos I didn't miss a payment.. Each and every month is me calling them to ask but they give me the same answer of errears.. I have never paid R450 which is our initial agreement and it has been a year now.. I am drained, I am swimming in depts cos I am unable to pay above R450.. At times I want to stop paying but at the same time I don't want to be blacklisted.. I am left with 2 years but I don't know if I will be able to keep up with this rediculous bills from vodacom.. I can't even recommend Vodacom to my worst enemy because they are big ****s