invoicing query

i received a R14500 invoice from MTN 2 months ago for data. I have an sky contract with unlimited data and i hardly use my data contract as i have wifi at home and only use my 3G contract when at customers who doesn't have wifi. i queried this with MTN as i have never ever used data over my limit. i paid them R4500 to cover my contracts cost to keep my lines on until the query is resolved. they then proceeded to cut my line. i phoned them and again logged a query. they said they will send me itemised billing for which they will charge me. they mailed me a link but no itemised billing. my contract have a R9000 limit and even with this they cannot explained why according to them my account was R14,5k I logged numerous calls with them but they dont get back to me. it is impossible to have and account to the value of R14,5k as my contracts doesnt allow for this. they have no interest in helping me sort this out. i have been with them for over 10 years and my account is always in limits. they dont investigate their own billing systems. i will make use of the Cell C offer to buy me out of my contract if this is not sorted and attended to. the service is shocking!