disgruntled client - appalling service

I signed up for an internet package in September 2014, upon receiving my bill on a monthly basis i ensure it is paid accordingly.This month i receive a bill with an amount due that is double my monthly subscription, when i inquired the amount at the branch i was told it is due to Telkom forgetting to charge me a monthly internet fee hence they are back dating my charge from Sept 2014. I thereafter attempted to lay a complaint as this was no error on my side, my account was paid monthly according to my bill and due to an error by Telkom my budget would now be thrown off. The best assistance i could get from the Telkom branch was that i should make an arrangement or repayment plan with no consideration of the impact this would have on my credit rating. I feel that my account was paid according to the bill i receive and Telkom have no right to now go and back date charges as far back as 6 months with no consideration for the impact this has on my personal situation.