Bad service everytime when i do upgrade

Why everytime if i did an upgrade i alway have a problem with cell c now its the problem becouse the old phone is using the old sim the big one not micro sim and i was told that im not going to pay anything when i call i was told that i failed one question which i dont understand becouse the invoice has correct addresses on the system is wrong bad service because when i went to cell c shop they told me that they going to charge me why becouse is the upgrade