Overcharged me, acknowledged it but still did nothing to fix it!

In my Nov billing I noted that I was overcharged on my contract as opposed to what I have been charged every month. I called Vodacom and after speaking to 3 people it was acknowledged that they failed to load my discount. I provided copies of my previous bills that showed what I am normally charged. They eventually acknowledged it and said that they will correct it in my next billing. My reference for my previous call assuring me of this was 1- ********** ********** . To my surprise, I have now received my Dec bill and guess what!! They have done the exact same thing AND have NOTcredited me for the Nov one either! I have sent them an email and still nothing! I have lost faith in calling their call centre as they LIE and assure you of something they NEVER deliver on! This is not the first time that they just treat you as a number, but when they want you to renew your contract, that are as sweet as honey, making promises from the moon and back. I am a client since 2002 and yet that means sweet nothing to them!