incorrect bill & *********** staff

I received sms notification from Cell C stating contract ends on 27/8/15 so I upgraded on 24/8/15. My upgrade contract was R499 and I set a R50 limit so at no given time should my bill exceed R550. When I received a high bill of R805, I was confused. Spoke to Aiden (rep at Cell C) who stated that I was charged a handset fee for my previous Samsung S4 since I've done an early upgrade as my contract only ends on 16/10/15 & therefore liable for handset fee for the next 2 months. Explained him the sms notification from Cell C & the delivery note dated 12/8/13 of previous contract which he dismissed & instead xferred me to the upgrade dept where I was told this is untrue since records indicate that my contract infact has expired and referred me back to bill query dept where a query was logged ***). A team leader called me later to say that the handset fee is not because of early upgrade but due to being undercharged 18 months ago & hence the handset fee being raised now. She further stated that no additional fee will be raised the following month. Guess what? just got another Invoice for R835. Very Unhappy.....