changed my contract while purposely misleading me

I was due for an upgrade last year around October. I wanted the note 4. I sent an email to MTN requesting to be on the waiting list for the first note 4s. I received a call from MTN not long after, to discuss the upgrade. I was told that on my current contract, the my choice 399, which has a set limit and if you run out if data or minutes you have to top up, I would not be able to get the note 4. I was told that if moved up one level, ie: still on the my choice package but the more expensive one, that I would be eligible for the phone. So I moved up to the R650 option. Now, a few months later, I find that there is no limit to my contract and that I am R2000 in debt because I was billed beyond what I thought was a set limit. It was not explained to me that I would not have a set limit as was the case on my previous package. I was led to believe it was the same principal, just the more expensive package. I was also billed R1750 for the month of January, where I should only be paying R650. Please help, this is a large amount of money I cannot afford to pay, it is not justified. I never phone, I have wifi at home and I work underground so I do not understand how the bill is so high