Possible ********** interest calculation

I called Wesbank's call centre to capitalize a prepayment, the service consultant (Lerese Swart) told me that it will only reduce the capital amount but not the interest. As this seemed strange I inquired what capital amount they use for the interest calculation. After asking the same question multiple times from different angles she eventually told me that Wesbank uses the original contracted principal amount. I could not believe this and requested her to verify the amount, she was evasive but eventually told me the amount which was the original capital amount. I have made numerous prepayments, but to Wesbank's own admittance they have been charging me interest on an amount that is significantly larger than what I owe them. I requested to speak to a manager, I was promised a call back but no one did. I called their call centre again, was met with very unhelpful service agent. Asked to speak to a manager (Ugeshan Naidoo), but he was unwilling to help me, and deflected my direct questions as being rude in an attempt to frustrate the situation so that I will not pursue my line of questioning. I called again, was promised that a manager will call me but no one did.