MTN ADVERT IS *********

I signed for a My MTN 10 gig contract in February, at Wonderpark Shopping Centre.Firstly, they gave me a wrong one, which was My MTN 20 gig, at a cost of R499. After seeing the error, I requested them to correct it , which they did and changed it to what I initially requested, My MTN 10 gig, in March. I am supposed to pay R299 per month as per the advert on MTN yellozone catalogue.To my surprise my bill for April came as R1562.58 and I have been calling MTN callcentre and no-one seems ready to can help or at least explain why or how.Whenever I call the callcentre they give me different explanations and I am here sitting with a bill of R1562.58. All over, if you look at the advert, it says MTN 10 gig monthly subscription is R299.I dont know if this the way of tricking us with false information or what. I request MTN to speedily resolve this because I am not going to pay that amount