overcharged with no valid reason given

good day , i upgraded my cellphone contract with MTN network in November 2020 , agreement was the monthly premium will be R475 , December month billing was correct , January R1165 , February R1140 , been calling their call Centre , no valid explanation given , all 4 cases i logged with them are closed without valid reasons or call explaining the overcharging. Even the store manager at Vaal mall promised to come back to me to give me the reason of being overcharged , its two weeks now waiting for the feedback.

kindly assist