My Limit has been removed without my permission

Hi<br> <br> I took out a contract in AUG 2015...the Contract was for a Samsung tabet with only 1 gig data (daytime) and 1gig data(night time)....I ensured that when I took out the tablet that there should be a limit and no calls could be made...so basically once the 1gig data is finished, then I'm unable to use the phone and can only top up if I buy airtime...<br> <br> All these months from AUG 2015...this has worked perfectly and has been the case...when I received my statement this month...too my suprise Ive been billed 1600 extra....when I phoned the called centre...I was told there's no limit on the account...but all these months there has been a limit....I'm not happy and will not accept these charges...MTN should be liable for this as the error lies on there side...<br> <br> Please help...I need this sorted out 2day before my account is debited .<br> <br> The phone number in question is ***.<br> <br> My email is delanoj@need bank. Co.za<br> <br> And my phone number is ***<br> <br> Work number ***