Vodacom False Advertising

About a year ago I was going to cancel my Vodacom contract that I had for a good few years. The Vodacom retention department then gave me a call an offered me a deal where I would receive a new phone, some airtime and the selling point, 1 Gig of data per month. A year into the contract the 1 Gig as moved down to 200mb, I have called the call centre twice already over a period of 2 weeks to which i have been told that a call has been logged. The only reason I re-newed the contract was to obtain the data, which i don't even have now. I really don't know what to do but I am certain that I will never have a contract from Vodacom again.