Vodacom over billing me for years

Vodacom has been over billing me for years now I need a refund. My name is Thomas Lewies cell no *** vodacom account ***9-1.iI have resently canceled my other contract due to this over billing. but stillthe over charge me. and they where giving me 5G but this month the add 3G it means now iam get 8G.and I didn't even agreed on that. I don't know why vodacom are it doing this to me. I tried to them many times but they can't even explain it to me why iam suppose to pay such a lot of money. they tell me a bout airtime advance. even if I have paid but every month I I will be charged for that . even if I didn't take an advance airtime. So please help me on this situation. I can't take it any more coz now vodacom is ****ming me and I have been a loyal and trustworthy customer for years.