mtn is trying to rob me

I had a 2 year contract with mtn which ended the end of march 2015.from April 2015 my sim was blocked and I figured its because the contract ended when I spoke to a sales person at signet terrace they said if I do not upgrade I must just pay R200 to change the sim to prepaid.I did not pay as I had another number that I was using.on the 17th July 2015 I got a call from mtn's lawyers telling me that I was handed over and I need to pay R4480.75 as my contract was still running even though it had ended in March.Now im told that if I do not pay my name will be put on ITC and I have no choice but to pay because theres nothing they can do.Aslam at signet branch told me on friday that I was supposed to call 808 to cancel but in April when I called another sales person they did not say anything about me calling to cancel.On tuesday 21st july I got another call from the lawyers telling me that the amount is now R4506.60. I refuse to pay this amount as I already paid so much for my phone and why should I pay for a contract that I am no longer using