Telkom overcharging me for 6 months

Telkom has been over-charging me by debit order for internet services for six months. This is because they failed to apply the agreed discount to my monthly charges. Telkom agents have confirmed that the charge is incorrect and for this reason. However, they have not corrected this error, which means that they now owe me around R6000. The last agent I spoke to said she had discussed the matter with her supervisor, but the problem is still not resolved, which means that the debit order at the end of July will likely once again be far too high. The reference number for this dispute is ***. My email address is ***