Upgrade not done three months later and being bill

I went into Clearwater Mall to do an upgrade on the 17th November 2015. This was never done on Vodacom's system until January 2016 - I was being billed incorrectly and received a huge bill because I was on the wrong package - I have been waiting three months for this to be rectified. The call centre can not assist me and neither can the store. The store manager William Tjatji promised to sort this out. Its been two months. He is always out of the shop when I call once I give my name and never comes back to me. What do I have to do to recitify this and I have paid my account in full on the higher amounts and have another huge incorrect bill due end of the month because this is NOT sorted out!