Not providing data that it promised

I recently renewed my contract in March 2016 but my 1st two attempts to renew did not go so well.So I called to cancel the contract as I wasn't happy with their service.I was then promised 500MB if I renewed the contract in addition to 2GB for 3 months that was on promotion with the phone that I took(Huawei P8).The agent that I spoke to at the cancellation department,Busi,told me that I would receive the additional 500MB every month as compensation for the initial bad service I had received and I would get it for the duration of my renewed contract(2 yrs).I received the500MB but now it had changed to a 5 month thing and 2GB wasn't sent.After numerous times trying to rectify this,I finally given 1,5GB,meaning that they just added to the 500MB I received at the beginning of March to male it to 2GB instead of 2,5GB that was initially promised.I am tired of calling and explaining myself over and over again to a different agent all the time and then put on hold for long periods of time without having my issue resolved properly.Vodacom has given me substandard service by not delivering the product promised and yet I pay a full amount each month.Kindly help..please.