MTN misleading customer into new contract

Got a call from a lady working at MTN. She said that i can get R110 airtime for R80 a month and if i decided not to accept i should return the package to the nearest MTN store. They will give me a reference number and return the package to them. Within a week the pacakge was delivered and a new sim was received. The lady did not explain to me that this would be a new contract. I took the sim card back to MTN Cresta. The guy that helped me called the company which is a division of MTN whose aim is to bring in more business. He was left holding on for 37 minutes after which he dropped the phone as I had to leave as I had other commitments to attend to. The guy at MTN Cresta could not take the sim card back so I could not obtain a reference number as was explained to me by the sales lady on the phone. I am sitting with a new contract in addition to an already existing contract which is not what I wanted. Firstly the lady who phoned did not explain to me that this was a new contract. She merely said that I would receive more airtime for my money. My query is that if the MTN stores cant return this sim card how am i supposed to cancel this contract especially if they let you hold on for 37 minutes abd this is not possible when we have to phone from our cellphones