Cant cancel contract

I tried calling the MTN call centre on Friday 22 May 2015 in order to cancel my contract and convert to prepaid as my 24 months has finished. (i.e. more than 7 days before the end of the month as recommended by the MTN store a few weeks ago). It was advised that the service was unavailable temporarily and that the call centre is closed (i.e. the 808 number). I tried again on Monday and Tuesday (25th and 26th May 2015) with the same thing happening. I thereafter called the MTN store in Westwood Mall (around 11am) and the lady there told me that there is absolutely nothing they can do and that the stores are unable to cancel contracts and that I have to pay for one more month. This is absolutely shocking behaviour from such a big mobile operator. It is completely unfair for me to pay for 1 more month of my contract due to MTNs inability to cancel my contract. It is also further concerning considering the fact that I hold shares in MTN and this is the way a company I have invested in treats its customers.