Over-billing and no correction after numerous calls

Took up a promotional offer as received by email to level up my contract plan from FlexOn 2 to FlexOn 4 at only R 50 per month additional charge. Ended up being charged almost R 150 per month extra.

Called numerous times to have the billing corrected and credited but to no avail. Keep getting transferred from one department to another. Requested a call back - which they did and undertook to correct. Never corrected. Requested more call backs to follow-up months later - still no fix to the issue. Currently been over charged for 4 months long.