Unexplained billing issue

I've been using MTN over 15years and I had little to no problems with them as a Prepaid customer but when I made the stupid decision to change my number to a contract sub that's when my issues started, from then till now its been a spiral down. When you ask them to resolve issues they tell you they don't know how to they need to contact a different department. My current is that I haven't been able to purchase data for last 4 months that is over R470 from usage, when I enquire about this no one has answers. It is now June, my invoice is out (that I didn't receive, I needed to call the call center) they tell me my usage is sitting at R1100 and some odd rands. I spoke to a manager to give me a break down as to how this happened, guess what she couldn't answer me. I got the same story I got from 10 agents that I spoke to in a space of an hour. MTN has the ****tiest customer care that I have ever came across with. No one knows anything and queries go unresolved. You can complain all you want, you will never get any assistance. I am so appalled and I will not advise anyone to use them or take a contract with them or you will have ongoing issues till your contract end with them. I am sitting with a bill of R2909.57 that I don't know anything about. MTN is good at taking peoples money without being able to give answers.