To MTN management

I have a problem with MTN store, i got a contract phone on june and our agreement was to pay 329 every month but instead they are taking 416. What happened is that last month on the 16th of november i got 2G + 25 minutes which is the package that i signed up for and they putted extra 300 minutes which i didnt ask for and they said i must pay 750 at the end of november...so i went to the man store to ask them why they've gived me that 300 minutes because i never said i want them and they said to me it won't happen again maybe its an error they will fix it of which they didn't do that. On the 17 of december they did again giving me those 300 minutes and i went to the store and they told to call 135 of which i cant get hold of them, now my bill is 750 again and is due on the 28...how can i pay for something that i didn't ask for? I really cant afford that much especially when its not my fault help me please what should i do or must i give them the phone back or what