FIX MY BILL!!!!!!!

I did an early upgrade in June of which I had to pay an Early upgrade fee of R350 which i paid, Mt contract thereafter was supposed to be R699 per months but the bill that followed was R1200 I paid that in July assuming that it was Including a pro-rata amount, August it came to the same figure again so I queried it as obviously I could not afford that amount as I budgeted for R699 as it should be, so I did not pay this month's bill and logged a query, But MTN is not resolving the matter, it instead has blocked my line due to non payment and has not resolved my query after logging two incidents, I'm fed up to a point that I will rather return this phone and go and get me a contract elsewehere. MTN IS NONSENSE