They don't want me to cancel my contract even when I done paying it.

Not good at all. I had a contract for two years and I finished to pay it. They still dedicated the next month and I called them to cancel the contract because I'm done paying but when we get to cancel the contract they dropped the call and then I called again still they do the same. Even when I go to telkom stores and they call at office, they still do the same. Now they suspended my sim saying I owen them R1200. The problem about this is that I value my phone number eish. Please help!