8ta / Telkom Mobile
Unable to give service

I have been happy with 8ta/Telkom Mobile, until I renewed my contract after 24 months.
I took the sim-only contact 4 option at a reduced rate, as per advertisement for R295 and kept my old phone. For the last 5 months they charged me full rates R500 and they are unable to correct the contract amount.
I have visited Telkom branches 4 times and called the call centre at least 10 times, lodged various cases and still nothing happens. Nobody phone back, no help or assistance from accounts.
I renewed my contract in January 2014, we are now in July. Is this reasonable?
They owe me over R1000 by now... by all the overcharging.
PLEASE HELP!