Angry but not surprised with Vodacom

If I could give them 0 stars, I would. In July I updated, knowing full well that I had a balance of R900 for my old handset, I chose an upgrade accordingly. When I went to do my upgrade the consultant informed me that the old handset fee fell off and I don't owe anything. I then took add ons thinking that I wouldn't be charged extra and I have documents to prove that I didn't owe a balance. They still billed me for everything including my old handset fee. I then took this to hello Peter and a consultant contacted me. He told me that he will be taking my word for the whole situation and gave me a credit of R900, he didn't even ask to see any docs. Anyway fast forward the next month, I'm only billed R402. 41( not surprised I just assumed they just subtracted that R900 credit) and I know I'd have to pay R1300.00 till I'm done paying for my old handset. This month, I was billed R1600!!! What is that? why am I being overcharged?