Change of Contract amount

I have 2 contract with Cell C. I have been billed the contract amounts up to untill this month were both contract the amount went up by R10. For example I was always paying R129 per month, now it has changed to R139 per month. When I call them they say its \terms and condition of the contract". When I ask which clause on the contract they could not explain. To me Cell C is practising unfair competition to other service providers. They advertise R129 X 24 and to me this should remain like this for theduration of the contract. They cannot change this amount other than the bridge of the contract. This amount is the amount that I have signed for not the other way around hence I have given them debit order outherization. Tommorow I will wake up Cell C taking R6Million on my account because of their term and conditions. I warn CellC customers to never take Contract with them because they can change the contract amount any time as they pleased. Other service providers the contract amount remain like that for the duration of the contract. I will never ever take cell C contract again for the remaining of my life."""