incorrect calculations

Good morning . I made an upgrade with mtn in may on the 25 and i was paying R409 on my previous contract , the current is R329 and i was told that im gonna start paying R329 from June . on the due date i called mtn asking about my current due amount and i was told to pay R409 ,when i ask why R409 because i made an upgrade ,the person who was assisting she told me that the upgrade is not showing on their system . July i did the same and i was told the same thing and she also told me to go to the store where i did my upgrade to enquire .on the 3rd of August my line was suspended and when i call the asking why my line is suspended i was told that i owe mtn about R578 including the penalty . i got several references all of them are telling me the different story . CAN MTN DO THEIR CALCULATIONS RIGHT AS I PAID R80 MORE FOR TWO MONTHS AND MY CONTRACT STARTED IN MAY NOT AUGUST ref ***, ***. ***. ***. ***