Very angry

I took out an additional contract on my name late in December last year. The promotion I had signed up for was that I would get 30 gigs( 10 gigs every month for 3 months) this was confirmed with the sale agent. Well well well to my surprise they allocated 30 gigs of data once the contract was activated and the whole 30 gigs expired at the end of January. This is not what I have signed up for. When i phone they tell me there is nothing they can do because that promotion ended in December. I even went back to the store where I got my contract from and the sales consultant confirmed once again that I was supposed to get 10 gigs of data over a 3 month period. She said she sent an email to MTN and to date I am still waiting for a response. I even still have the catalouge that shows 10gigs for 3 months. Now MTN want to get out of giving me what I was supposed to get. I want this resolved as I believe from the consultant that I am not the only one that this has happened to.