BIS Being cut

last year in Jan my contract was due for an upgrade, I was not going to up grade the contract as I had no need to, I was call by some one in upgrades dept and offered a top up contract and the only reason I took the contract was that I was offered BIS @ R4 a month. but since the change over of contract Vodacom has constantly been cutting my BIS every few months n then I have to call Vodacom n explain my prob to get them to reactivate my BIS. this month once again they have cut my BIS and I am so frustrated wit them I am at the point I actually want to go the complains commission as this happens so often to me it is ****ing me off. I even have a invoice where it shows I will be charged @ R4 over 24 mnths for BIS. WHY can Vodacom not honour their contract as I honour my part n pay them every month.; I am now at my wits end and not sure what to do as I have a year to go and do I have to call them ever so often to fix their own mistake.