MTN (PAROW BRANCH)

As a student I took out a cell phone contract with MTN four months ago. I had the phone for three days and as the contract states in seven days if I am not satisfied with the product I can return it. I gave them my reason why I am not satisfied with the product and they gave me a cancellation form which I completed and they took the phone. By the end of the month they billed me and I made contact with the Call Centre and explained my situation and they said that they haven't received any cancelation application. After a month they contact me to say that I must fetch the phone and said that there was nothing wrong with the phone and I cannot cancel. Numerous times I went into the branch (MTN Branch Parow) asking them to assist me, but no success. I had network problems with the phone since day one. Today I went back to the store and they said I must take the phone to a cell phone store they cannot assist me. Their service is terrible!!! I did not want to take the phone to a cell phone store as I might lose my warranty on the phone. I even port my number to MTN but the network is still giving me problems. I Am unable send and receive SMS's and unable to make and receive phone calls. Please assist as I do not know what to do.