I want to cancel this contract and want my money back

MTN is selling people a lie and their so called consultants have no clue of what their doing. i took up a data contract in August and was suppose to receive 40g of data and later discovered that they have been giving me 20g of data while making me pay for 40g. I was assisted by a lady called Nelly at the MTN store in menlyn mall and when i called for follow ups i'm always told that shes on leave. i also spoke to Sammy who gave me wrong information and referred me to the customer service only for me to be told that Sammy is the one who needs to assist me. The staff is *********** and unprofessional because when i called again they put my phone on the side and ended up not taking my calls.