Termination of contract

Good day Peter Can you please help me with MTN South Africa, I bought cellphones on contract for 24 months period at the end of July was my last payment and I went to their branch to terminate the contract since am done with my premiums, they told me it can only be cancelled on the 4th of August since then I have been struggling with them they don't want to terminate the contract even if I call their call center they do not pick up in the cancelation office.