*****ing from me by double charging

Vodacom has been ****ming me money through the so called airtime advance, I am on contract with them for years so out of nowhere and without any communication they decided to add the advanced i used on my installment and I noticed that my installment is not fixed now bt did not pay much attention to it m, so this month i pay my instsllment they come back to say i am owing, I call them to ask what i am they said i am owing for airtime advanced and i told them to check on my account to see if i owe any advance because the advance is and has always being deducted from when i recharge airtime and the assistant failed dismally to explain why i was paying double for the advance and asked to send my bank statement as POP showing my payments and whatnot which I still dont understand why they want that because everything shows on their screens, they can see when i load airtime snd how i use it and when they ***** it after collecting money with my contract statement, I was on call with them for over 2 hours they still couldnt explain where is the amount i am owing is coming from cz they cant say advance cz It was deducted from my airtime....THEY HAVE BEEN *****ING FROM ME FOR MONTHS, I WANT AN EXPLANSTION, THEY TOOK MY BANK STATEMENT SND RAN NEVER TO CONTACT ME AGAIN YET THEY SEND ME MSGS SAYING I AM OWING