Telkom overcharging ADSL

I upgraded my ADSL line from a 20GB monthly data usage with 4 Mbps line at R384 pm to a 50 GB monthly data usage with a 20 Mbps line at R601 in December 2014. They are charging me R768 for this ADSL line and data. I have tried numerous times to call the call centre and been to Telkom direct outlet but they haven't been able to acknowledge the issue. The have only credited me once off the R167 overcharge on my February Bill.