Over charge on bill - blocked lines 5 months in a row

Mtn has over charged me after doing an upgrade in October 2020. They have acknowledged that they have over charged me, but they have not credited me up until now (April 2021). Every month since December they will block my phones as they are requesting that I pay them an amount of almost R500 which I do not owe them, on top of my actual monthly bill which I pay in full. If I do not pay the R500 they block my lines. I then have to call customer care and explain the entire story over and over again. The answers I get from customer always changes and are as folllws: - the billing department can only help me and they cannot transfer me to them - the billing department will credit my account but they can't tell me when - they not sure why the billing department does not want to credit my account even though they acknowledge the over charge - they can't help me unless I give proof of the upgrade I did, they domt have that information.

I've been blocked close to 10 times, speaking to 1p different people and 2 managers and NOBODY was able to help me. They quick to request payments fr their customers but cannot provide a decent service in return. I have no idea when this query will be solved, and nobody at MTN can seems to be able to assist me.